So, you witness a debate with some friends on which length gas system is best for the and you think to yourself “huh, what’s the difference?”. We did a real deep dive for you this time. Why is your AR-15 gas system choice so important? In short, if you have the wrong length of gas system for your barrel, it can cause malfunctions or excessive wear. Stick with us and we will break everything down below!

What Gas System Does The AR-15 Use?

Let’s start by asking Wikipedia:

“Gas-operation is a system of operation used to provide energy to operate autoloading firearms. In gas-operation, a portion of high pressure gas from the cartridge being fired is used to power a mechanism to extract the spent case and chamber a new cartridge.”

So on an AR-15 specifically, there is a small “gas port” (small hole) in the barrel that vents gas with every shot. This gas travels through the gas block and gas tube into the receiver, where it powers the bolt carrier group and auto-cycles the next round. This is known as a “direct impingement” system.

The GIF below from Wikipedia demonstrates how the gas travels:

AR15 Gas System

Why Does the Length of the Gas System Matter?

The length of the gas system (length from the receiver to the gas port) should increase as the barrel length increases. The reason has to do with “dwell time” – the length of time that the bullet is in the barrel after the shot is fired.

On a longer barrel, the dwell time will be longer, because the bullet has to travel a longer distance. Specifically, there is a tiny amount of time when the bullet is traveling through the barrel and it is past the gas port, but it’s still in the barrel. During this time, there is hot gas traveling through the gas tube into the receiver. As soon as the bullet leaves the muzzle, the gas stops flowing.

If there is too much barrel length after the gas port, then too much gas flows into the receiver and it can causes issues with excessive recoil and wear on the rifle.

If there is not enough barrel length after the gas port, then too little gas flows into the receiver and the rifle may malfunction.

More of a visual learner? We got your back!

What is the Correct Gas System Length for my Barrel?

The chart below shows the typical gas system lengths (distance from the receiver to the gas port) and how they pertain to barrel length. As you can see there are wide ranges here, typically the closer the barrel length is to the middle of the range, the better it will function (for example 14” barrel with a carbine-length gas system). Something to remember is that the right gas length is a set value and it is highly dependent on the barrel being used. We highly recommend looking at the barrel manufacturer’s specifications regarding their suggested gas system.

SystemBarrel LengthPort Distance
PistolLess than 10 inches4 inches
Carbine10-18 inches7 inches
Mid14-20 inches9 inches
Rifle18 inches or more12 inches
We highly recommend looking at the barrel manufacturer’s specifications regarding their suggested gas system.
Pistol Length Gas System

AR15 Gas System Considerations

It’s worth mentioning that there are factors that can affect the gas system, beyond the length of the system. If you have reoccurring issues and you suspect your gas system is to blame, check out page 110 in this FREE US Army Manual for the AR15 / M4 / M16.

Adjustable gas blocks – Some gas blocks are available that allow you to throttle down the amount of gas that travels through them. For example, you can constrict the gas to the point that the rifle is a single shot.

Buffer weight – The weight of the buffer can have a big effect on how the AR15 cycles. A heavier buffer will generally require more gas pressure to cycle, so it is possible to moderate the effect of too much gas pressure and smooth out the action by using a heavier buffer, or vice versa with a lighter buffer. Buffer springs can also have an effect.

AT3 Tactical Buffers (Carbine, H1, H2, and H3)

Ammo type – The weight of the bullet and the powder in the cartridge can make a big difference. A heavier bullet may travel more slowly and cause a longer dwell time, whereas a low-power cartridge may not build enough gas pressure to cycle the action properly.

What is a Piston System?

While you poke around on the internet looking for answers (we got em) you may stumble across the piston gas system that has made a resurgence in the past couple of decades. Piston systems operate on similar principles as direct impingement in that there is a hole in the barrel that vents gas into a tube. The big difference is that inside this tube is a piston that either strikes a bolt carrier group (BCG) or is attached to one. Notable firearms that use this would be the M1 Carbine, M14, and AK47.

The GIF below from Wikipedia demonstrates how the Short Stroke Piston Gas System Works:

Piston-driven systems (long stroke and short stroke) are nothing new and have their apparent attributes such as not relying on a buffer system to the rear. This eliminates the need for a fixed extension of the lower receiver.

Their critics will make sure to note that these types of systems are not only heavier on average but also the system is not standardized. This means companies who make guns with this system or parts for this system, will have proprietary designs. Nothing will really interchange as far as the gas system is concerned.

Ideally, we will cover this topic separately at a different time. We just wanted to hopefully sever any confusion about its lack of in-depth inclusion.

What Length Gas System is Better? Mid Length vs Rifle Length vs Carbine Length Gas System

The most agreed-upon consensus is the longer the gas system the better overall. The main reasons for this would be that there is less wasted powder, velocity, and energy. More positive attributes include a smoother cycling feel and lighter felt recoil. Even though carbine length is more common off the shelf (as far as stock guns) it is accepted that both mid-length and rifle length are very comfortable in comparison to their shorter counterparts.


Do those things make the mid-length and rifle length better? No, not necessarily.

Carbine length is more common on shelves because it is not only a shorter more maneuverable option but it is a sort of happy medium. It’s versatile! It fills most roles to an acceptable degree. Going with a shorter or longer gun would lessen the roles it could easily fill. In short, carbine length is a do-all.

“Better” is relative. Do you want a more comfortable and efficient shooting experience? Maybe go with the longer options. Do you want a close-quarters home defense gun? Try the pistol length. Do you want one and done? Roll with the old faithful carbine length. Pick which one is best for your intended role and you will never look back thinking you should have got something “better”.

AT3 Tactical AR15 Gas Tubes (Rifle, Mid, Carbine, and Pistol)

One Last Tip

If there’s anyone that knows the AR-15 platform, it’s the US military. As a special offer for our readers, you can get the Official US Army Manual for AR-15/M4/M16 right now – for free. Click here to snag a copy.

Read More

112 thoughts on “Mid Length vs Rifle Length vs Carbine Length Gas System [2024 Update]

  1. Suprised no one said it but there is a big exception to this. 300 blackout should use pistol length gas system regardless of barrel length due to how the ammo burns. It will not cycle like it should. I see people ignorantly put carbine on their 16in and see all the issues they have with performance and there is a reason. Also, you will not be firing sub 300 on a carbine system. The whole gas length system is usually the case but there are cases where you have to do your gas system based off the bullet type. Just sayin. Source im big into gunsmithing.

    1. With that said you could put an adjustable gas block on a carbine system with a 16in barrel but youre still better off using pistol length gas system for 300 blackout even if you have a 16in barrel.

  2. […] heated at temperatures of 20–70 degrees Celsius, it transforms into gasoline. Propane is a medium-length gas with a temperature range of 5 to 7 degrees Celsius and a vaporized state. Because gasoline contains […]

  3. Never mind. I just realized my error. My two were purchased as pistol kits and utilize braces rather than stocks. I was using the term SBR improperly.

  4. Good read.

  5. I’m looking to build a AR in 12.7x49mm with a 28″ barrel. I keep getting conflicting information on which Gas system to use. The manufacturer says to use a mid-length gas system but my calculations say to use a rifle length. I’m more prone to trusting the manufacturer and going with the mid-length system. Anyone have a clue if you can get a mid-length gas rod custom made and if so where?

    1. By manufacturer are you talking barrel manufacturer? If so I’d ask him why the mid and not rifle length. Reminding him you’re talking a 28″ barrel.

      Picture in your mind the instant the bullet has just passed the gas port. The problem I see here is the length of the gas system needs to be short enough for the gas pressure/volume to be high enough to operate the bolt. While at the same time keep enough pressure/volume to get that bullet the rest of the way down the barrel. For the 5.56 and the various 30cal weapons the gas system lengths are fairly standard. But for something like you propose I doubt there’s a lot of data out there for it and you may be in for a lot of experimenting to get things right.

      As for a custom gas tube, if you end up needing one (a mid length tube is a mid length tube regardless the caliber of the gun). Any competent local gunsmith should be able to bend on up for you. Then again it’s not far off from what the local auto brake mechanic does. Pick up a tubing bender for the size tube and bend your own.

  6. Queries about gas system length? Part of the answer lies with the caliber chosen almost regardless of barrel length desired. The longer the gas system, the more chamber pressure declines which is applied to the BCG for shell ejection. Calibers like 7.62X51mm and 5.56mmX45 fall in the “normal” range for generic gas system lengths. These two calibers and there equivalent clones have the majority of market offerings. OK, what about going to the AR caliber fringes to the .243 Winchester and it’s clones? That’s an old and boring cartridge, right, but oops, gas system problems. This cartridge, though, boring these days, is what’s described as over-bore – same powder propellent amount as it’s parent pushed through a smaller hole, resulting in an extremely high gas volume in relation bore size. Though the .243 Winchester goes with the 7.62X51mm platform, it’s propellent gas effluent overwhelms today’s standard gas offerings for it’s parent resulting extreme over-gas conditions. How about ripped off rims which leave the cartridge stuck in the chamber? What to do? Even buffer weights of 8oz don’t totally cure the problem and adjusting the gas block to almost closed works but to use a buffer weigh that heavy requires a special length buffer tube if carbine is the goal. The real fix? Look to what some online builders are doing for 6mm Creedmore – add length to the gas system, perhaps 3″ so a carbine platform .243 Winchester (similar to 6mm CM) would do better with a rifle length (or longer) gas system. Gas port diameter was mentioned by one online builder as another feature of the gas system which may offer adjustment, though references to that issue are scant or non-existent. So how does this info come to me – a carbine 18″ .243 Winchester build which has gone through many adjustable gas block, many buffer weights (now 8 oz) and springs and a specialty longer buffer tube, gone from ripping off cartridge rims and difficult extractions to over-gassed but working. Next step is perhaps going to a rifle buffer tube with an over-weight buffer. This over gassed condition has resulted in an over-weight rifle. Lesson – match the gas system length to the cartridge, the proper fix for the .243 Winchester (and similar) is a longer gas system or smaller gas port.
    Here are two web sites which show some insight into chamber pressure:
    Scroll to see the chart

  7. What is better for my Ar15 with 16″ barrel and that size incudes the a2 style muzzle break.
    A mid length gas system or carbine system and do I have to change the barrel ?

    1. Adjustable buffer from Odin Works is an easy way to switch on a normal basis. Shorter gas system, heavier buffer. Visa versa. Although the mk2 system from BCM is another option that is a bit longer tube with a customized (longer) carbine style buffer with a rifle spring. More reliable on multiple types of ammo & less felt recoil. Easiest fix, Adjustable buffer.

  8. I’m looking at a 16” upper with a carbine length gas system. I’ve been shooting with a mid length system upper on my lower, and am wondering if I will need to switch out my BCG or buffer/spring for the shorter system. Same caliber, so more asking about weight.

    1. The only thing you may have to address is the buffer weight. If you’re just going to carbine length system from mid-length, you shouldn’t have any problems, and should not need to change anything. BCG and Charging Handle is the same. I switch between the two on the same lower quite frequently.

  9. Has anyone had any issues with the polymer 80 mag well and mag release is not catching the mag . I’ve tried everything I can think of to narrow down

    1. Lighter buffer weight &/or a reduced power Sprinco spring should do the trick.The Odin Works adjustable carbine buffer is pretty great for different weight ammo if any other cycling issues arise but I would imagine a reduced spring will probably fix your problem alone. Good luck!

  10. 10.5″ BCA upper with carbine gas port in 350 legend, shooting Winchester 145 grain round with side charge action and with 3oz buffer, round loads from mag ,fires, and jams with 70% of brass pulled leaving 30% of brass still in chamber, making it a single shooter, what am I doing wrong on this set up .??

    1. Sounds under gassed. Try an adjustable gas block, or lighter spring and buffer, or a lighter bolt carrier group. You’re now in the tuning phase.

      1. If he has a standard gas block then an adjustable block will only serve to reduce the gas. They never increase the volume over the stock units (assuming equal port sizes obviously). A standard, commercial gas block is effectively “wide open” or max gas every shot.

    2. did you ever get a solution to this issue?

  11. I’m going to put together a 6mm arc and noticed that CMMG lists the gas system as rifle length on their 16″ carbine barrel, why is that? What’s even stranger is on their complete uppers they list their carbine in 6mm arc as having a carbine length gas system as opposed to the rifle length gas system on their 16″ barrel sold separately.

  12. I’m building an AR15 rifle with a 24” barrel and was wondering what size gas tube do I need?


    2. You will need a rifle buffer, tube and spring on your lower. The manufacturer will have the gas port as a rifle length gas system. You’ll then need to have the correct size diameter gas block for the barrel. Hope this helps.

  13. So ive started building a bit… have pushed out a couple of pistols with 7.5 in barrels and pistol length gas tobes, .750 non adj gas blocks, everything else ( buffer, spring etc) is “pistol weight” also.. barrels are .223 wylde… both weapons are having issues cycling… there are no obstructions in gas sys, dry cycle live rounds of .556 do fine, but not .223… also when firing; neither .556 or .223 cycle properly… one shot then jam due to: spent cart does not eject fully/ properly… noticing that upon working the bcg back and forth with oil the 223 rounds now actually pull out of the * a little bit like they should, just not fully ejecting… checked actual ejector on bolt and all looks fine.. spring, pin, and plunger/o ring all in place… any advice?! Help?! Please advise.

    1. So sounds like you might be undergassed. Welcome to the world of AR pistols. You are now in the tuning phase. You can try a lighter spring/ buffer piston, the cheapest way, or try an adjustable gas block, same price but more work, or go to a lighter weight bolt carrier group, easiest but most expensive. The truth is that every pistol is pretty much an experiment, unless you get an exact parts list from an established armorer, and even then, its rare not to need some tuning to make it cycle. That’s the fun of the whole thing. But also the frustration, just when you think you’re done, you’re not.

  14. Have a 10.5 inch build came with a pistol length gas tube in it? Should I replace with carbine length? Will it hurt it?

    1. You are going to replace your entire barrel just to go with a different length gas system?! Where the gas port is drilled on the barrel determines gas system length not the tube!

    2. It came with a pistol length tube because it is a pistol. If you put a carbine length buffer tube that can accept a stock on it, and you own said stock, you may be in “constructive possession” of a SBR which, unfortunately, is a federal crime unless you possess a NFA tax stamp for it.

      1. he’s talking gas tube and you are talking buffer tube.

  15. I’m currently planning my first build, which will be a long-barreled (14-14.5″) pistol, due to the letter of my state’s law. I was thinking of using a mid-length gas system; but am concerned about it being undergassed. I will be chambering in 5.56mm, and will likely be using 55gr & 62gr FMJ & JHP. I don’t currently expect to be using 77gr Black Hills; but would like input as to whether it’s feasible to set it up to be reliable for all three without resorting to an adjustable gas block. Like I said, this will be a first build, so I am trying to research the pieces before I start buying parts, so I don’t wind up with a useless pile of pieces that can’t work together.

  16. Can i put a 10″ 308 pistol length gas system upper, on a dpms lower a2 rifle buffer and stock system? I know it will fit, but will this work? What r the potential issues?

    1. issues can be is that will be a Short Barreled Rifle a NFA item that can get you in trouble with law enforcement/BATF by putting a barrel with length under 16 inches on a registered rifle lower or lower with a rifle stock.

    2. NO…once a rifle always a rifle. If the lower receiver was transferred or configured as a rifle it is forever a rifle. Adding a 10.5″ will not make a pistol it will make an SBR, unless you plan on permanently pinning a 6″ muzzle device to it BEFORE mounting it to the DPMS lower.

  17. I’m currently building an 8.5″ pistol build in 6.8 SPC. I’m looking to use a pistol length gas tube and a carbine buffer although which weight should I use for the buffer? I’m getting mixed answers I’d assume I should use an H or H2 buffer?

    1. Your going to be stuck with the gas tube length that your 8.5 inch pistol barrel is supplied with from the manufacturer. Barrels come machined for one lenght gas tube period, end of story. It will probably be pistol length. You will want a pistol buffer tube, and get the spring and weight as a set. You can play around with the piston weights and the bullet weight if it doesnt initially cycle to your satisfaction. You can use the carbine length buffer and spring but dont even think about putting a stock on it, unless you do the paperwork and pay the 200 before you pass go. Start with the h2 buffer and work from there. With pistol lengths and the more esoteric calibers, the whole thing’s an experiment really and it’s going to require tuning most likely.

  18. If I use a 20 inch barrel what type of gas tube, gas block, and buffer tube should I use?

    1. The gas tube should match the barrel. The gas block should match the feed ramp built into the upper receiver. If it’s an M4 feed ramp in the receiver then use an M4 gas block. If it is a rifle feed ramp then use a rifle gas block. The buffer tube and buffer should also match each other. If you have a collapsible stock then the buffer tube is shorter than on a fixed stock so you can’t use the long rifle buffer. It will keep the bolt carrier from coming all the way back. Use a short carbine buffer for carbine tubes. The stock doesn’t care which upper is on the weapon. It does matter that the buffer and spring match the buffer tube length.

  19. It’s hard to ϲome by well-informed people in this pɑrticular topic, һowever, yoᥙ seem like you know what you’гe talking about!

  20. Needing opinion

    Looking to purchase a 300 blackout barrel to swap from 5.56 to 300blackout. Barrel will be 16inch, what gas system length will be the best for hunting rounds being 100-200 grain charge ?

    1. Use the same gas system you have now or just re-place the gas block and tube that is required for your new barrel. Your present BCG and CH will work just fine, no change there. The bullet weight for hunting will depend on what you’re hunting, light bullets for small game and heavier for larger I think should be the way to go. You can even use your 5.65 barrel nut and handguard for the swap.

  21. One thing I notice is most people do not try the mid length uppers with an A2 lower. I have built three of them using the rifle receiver and rifle buffer and they work very smooth. The same uppers on a carbine lower work just fine.

    1. I built a 16 inch barrel with a mid length gas tube and matched it up to an A2 buffer system, looks awesome but am having troubles with it. It will fire 1-2 rounds then jam ,Failure to close bolt fully. There is a round in the chamber but its like he timing is off.

    2. I’ve had the same results with 3 mid length builds and rifle buffers

    3. Are you using a rifle spring & the actual A2 buffer? Those systems are specifically for a rifle length spring & a special buffer that longer than a carbine but shorter than a rifle buffer.

  22. Great article. I would suggest adding a section on the use of suppressors and how they affect timing and gas pressure.

    1. Hi Mike,

      Thanks for taking the timeout to read our articles. We will look into adding a section on the use of suppressors. Until then, we will continue to produce content that would benefit AR15 owners.

      All the best,

  23. I would like to challenge one of your measurements. For the A1/A2 handguards to fit you need at least 12 & 1/8th inches from the front of the barrel extension flange to the shoulder on the barrel for the front hand guard cap/back of gas block. Add another .13 inches for the flange it self and another.38 inches from the shoulder to the port center and the distance from front of receiver to gas port should be closer to 12.435 inches.

  24. I want to switch my bushmaster varminter from direct impingement to piston I would like to know what the length of the gas system is on the varminter

  25. I have a 16″ barreled 6.5 Grendel upper, which I bought an 18″ 1:8″ twist barrel to replace the original for the added velocity. The port on the new barrel is positioned for a mid-length gas system; is this going to be hard to tune in comparison with the original system? Any advice would be helpful. Thanks in advance!

  26. i have a colt LE6920 with a 16 inch barrel. i am looking to shorten barrel length to between 11-14 inches. what is the best mil spec barrel to buy and and what concerns related to the gas chamber

    1. BCM makes a nice 11.5″ upper assembly, or just the barrel. Mine has worked 100%.

      1. Order a complete BCM upper or parts. If you part them out it will cost more to build yourself. Get it built and it will be tuned perfect with the hand guard of your choice, proper gas system, optional bolt carrier, and charging handle. BCM is always perfection you can not go wrong. Spikes also does a great job with complete uppers or parts.

  27. I have a new build ar15 using m4 setup but a 20″ barrel with a rifle length gas system. I have a failure to extract. The bolt doesnt come back far enough to unchamber the spent cartridge. I have ruled out the BCG. The std spring and buffer are used and ive checked the position of the gas block. What could be the problem??

    1. I have the same issue on my AR build, what worked for me is I installed a adjustable gas block to mine as it was just needing a bit more pressure coming back from the barrel and the standard low profile gas block did not have a big enough hole to let pressure through so the adjustable one worked just fine.

  28. Hello,
    I have a 16″ 300AAC rifle. I’m purchasing a suppressor and handload my ammo. I’ve got the loads down to subsonic (around 950 fps). I want to file a form 1 and cut my barrel to 14. How will this affect the gas flow/cycle with using the subs with such little powder. Thank you

    1. You should have an adjustible carbine gas system or pistol length regardless on 300aac. It is one those rounds where the whole conventional rules doesnt apply like other calibers. It will burn fine from a 14-16in. Youll get about 200-300 extra fps. Anyone tells you otherwise hasnt a clue what theyre talking about. Source: gunsmithing

  29. tube length for a DPMS LR308 upper with a 16 inch DPMS barrel?

  30. Who cares, and no it won’t cycle with the carbine buffer and rifle upper. You need a H-14.644 buffer from the mini 30

  31. Building my first ar15 firing 6.5 Grendel. I have a 20″ Shilen match barrel and need to know what length gas tube and hand guard to use. I have a 15″ currently.

    1. The barrel manufacturer should have listed the type of gas system, most likely a rifle gas tube on a 20”bbl

  32. I am wanting to put a long quad rail on my dpms oracle, it has the 6.5 handgaurd that is now an mfg quad but my accessories are getting a bit snug.

    My question is, i have the large gas block with the rail on top and want to know if the quad rails from say midwest or troy with the cut out for a front sight will go aound the gas block rail? Is it a waste of time? Should i just mount a low pro block and full float and entended to cover it?

    1. Extended 15 in drop in hand guard goright

  33. I’ve got the carbine 15. What do I need to change my 7″ handguard into a 10″ handguard tube?

  34. what do I need to change my carbine length AR to rifle length?

    1. Longer barrel.

    2. Rifle length gas tube. Rifle length barrel(20”). Probably need to swap your buffer and spring as well. Rifle buffers are heavier and springs are longer.

  35. I just bought an Adams arms rifle length 16.5″ bbl what length of hand guards do I need

  36. I purchased an 11″ barrel for an AR pistol build only to find there was no gas port drilled. Do you know the diameter of the gas port and what are your thoughts on installing an adjustable gas block? Thank you.

    1. Pistol barrels are different than normal AR barrels in that there is no gas port which means no gas block is required. This is because PCC ARs are direct recoil operated, not gas operated. As such there are a few things different about building an AR9 clone.

      1. You are referencing Pistol Caliber Carbines, when not all AR pistols are 9mm or blowback actions. A lot of AR pistols come in 223/556 with DI gas systems.

  37. I just bought a 14 inch rail for my ar 15, it’s a 16 inch rifle. What do I need to buy to be able to put the rail on? And what steps to take to put the gas block on? What gas block? Or any other items I must buy to put the rail on and change the gas system

  38. hello I am trying to put a low profile piston operated system on my Smith and Wesson AR 15 carbine linkth rail and a 15 inch free floating rail after I install the piston kit what kind of piston kit do you recommend thank you

  39. Rifle length gas system 18″ to 20″ barrels

    I use a Carbine lower M4 Style stock

    My Buffer is a H6 Damaged Ind. Buffer (5.2 OZ) and Enhanced Carbine CS spring on my MK 12 with a LMT Sopmod stock rifle length gas system

  40. I just built a mid rifle but used a carbine buffer. What I’m seeing is the bolt carrier does not fully retract and once in a while it will not cycle the next round and it does not Lock open when empty. Any thoughts?

    1. The carbine buffer is a red herring, unless it is too heavy, that a will slow things down. It sounds as if you are under gassed. Make sure the gas block is properly aligned.

      1. In addition to other issues outside of the weapon itself. Ammunition plays a big role. Firing .223 instead of 5.56 or weak by manufacture ammunition. Just a thought

    2. Make certain your bolt catch, bolt catch plunger and bolt catch spring are properly installed. You should have already cleared the rifle chamber. Insert an empty mag with the bolt carrier closed then cycle the BCG with the charging handle. The bolt carrier should have been stopped, chamber open. If not, two things could be wrong. First, the mag has a problem with the mag cartridge follower being worn or otherwise damaged (this is most likely the problem) or second as someone else said, the bolt catch could be damaged. I don’t think that would be the case because the catch is made of pretty good steel. If the weapon is cycling fine, I would try a different mag because, with proper cycling during firing, I would doubt very much you have a buffer problem.

      How about posting a reply to what you find out.

  41. John,
    Wrong answer, Bolt carriers are the same length, There are 2 profiles semi auto and full auto. The semi has a shorter bottom extension at the rear and is therefore lighter than a full auto. My semi auto in my Bushmaster varmit rig is the same length as my BCM full auto carrier.

  42. Your bolt carrier group not locking to the rear when the magazine is empty may be caused by a worn or low quality follower in your magazine. After the last round is fired, the magazine spring should push the follower to the top of the magazine. At the same time the tab on the bolt catch should contact the back of the follower and be pushed upward. As soon as the round is fired and the bolt carrier group moves to the rear, the bolt catch should be in the “up” position. This will keep the bolt carrier group locked in an open position.

    Worn or poor quality followers can fail to push the bolt catch up, causing this problem. I suggest upgrading your followers with Magpul followers, it will make your rifle more reliable.

  43. If I use a 10″ barrel to build an AR15 rifle what size of gas tube will I need?

    1. Depends on caliber….If you’re building a 5.56 or Wylde chambering, a carbine gas tube should work just fine. If you are putting together a 300AAC Blackout, then use a Pistol length gas tube.

    2. If your using a 10″ barrel your not building a rifle, your building a pistol.

    3. Don’t forget about the $200 tax stamp needed for an SBR….

      1. $200 tax stamp needed for an SBR….? Where do you live that you need that? I just bought 2 SBR kits from PSA. I didn’t have to pay for any tax stamp. Only need one of those if I want to put a hush can on the end of the pipe.

        1. Never mind. I just realized my error. My two were purchased as pistol kits and utilize braces rather than stocks. I was using the term SBR improperly.

        2. If you are building an SBR you need a Tax Stamp regardless of where you live. What you bought from PSA was intended to be used on a “pistol” build, case closed. Tell you what, take your “SBR” down to your local law enforcement or BATF office with a collapsible stock on it and a barrel less than 16″ and see if they tell you that you need a tax stamp, hopefully you look good in an orange jumpsuit. The difference between an SBR and an AR pistol boils down to one thing; a Short Barrel Rifle has a buttstock while a pistol does not (although the “brace” for a pistol is nothing more than a loophole). The AR Pistol is not designed to be fired from the shoulder, while the SBR is. The AR Pistol is 100% legal for any non restricted American to own while the SBR requires special paperwork for every rifle issued by the ATF.

  44. The carbine and rifle bolt carriers are not the same. I have a lower receiver with the shorter buffer tube and buffer and a 16″ upper and 20″ upper. There are no issues with cycling on either, but the bolt carrier on the 20″ upper will not lock back when the magazine is empty. I still have to contact the manufacturer to find out if it can cause any damage. I was trying to do the same thing and only have one lower receiver, but I will probably start building a rifle lower receiver soon.

    1. What are you even talking about?? It’s literally the same bolt carrier. There is no difference what so ever.

      1. He’s just saying it wont lock back not that he needs to do anything with the bolt carrier. Definitely sounds like a buffer issue with a carbine gas system and a 20 inch barrel. Guess he didn’t know that on the 20 inch barrel he needs a mid length or rifle gas system. Or replace the buffer on the 20 inch but due to the carbine length it will probably where extremely fast.

        1. Helpful thanks for not digging on the inexperience of others!

        2. Len’s post is nonsense. 20 in barrels should not have a carbine length gas port, and it’s doubtful and company would manufacture one. There is also no buffer in the 20 inch upper, he’s using a common lower with 2 uppers so changing buffer tubes is impractical.

      2. You are correct sir! Some folks are smoking crack on here and have no idea what they are talking about! Lol!

        1. Isn’t that what forums and discussion are for? For those who might know to help others? In my experience those who trash others’ ignorance on forums are the ones who don’t know anything and resort to unproductive commentary. Shame on those persons you are part of the problem and not the solution. Exhibiting narcisim to make things about yourself. I hope this is not a practice we start to accept as normal Captain Obviouses. Of course the individual doesn’t know thus the question! We can do without the informing us of this situation please.

        2. With you on that bro

    2. I may be wrong but that 20 inch barrel will need a different buffer tube and that may fix the problem. Or you could buy a mid length gas system and avoid the buffer tube all together. I had a problem with one of my ar’s not cycling properly with a silencer and had to get a new buffer tube and spring and haven’t had a problem since. Like it says above, 20 inch barrel needs a mid length or rifle gas system. A carbine will be to light without a new buffer. Speaking from experience only

      1. Speaking from no experience, considering that explanation. Barrels are manufactured with only one gas system possibility, you cannot buy a different length gas system or “avoid the buffer tube all together”. Please do not post when you do not know what you’re talking about.

    3. This is my first rodeo. I want to build my AR15 with a 16″ barrel. Which gas block would be better, carbine length or mid length?

      1. Carbine length or mid length is the gas tube, not the gas block. The manufacturer of your barrel will let you know if it takes a carbine or mid length gas tube. They will also let you know the size of gas block you need. ie. .750, .625. etc.

    4. Just fyi, check your mags. different runs of the same manufacturers’ mags can cause a similar issue. (ex. I had a variable no-Lockback issue, and it was a specific run of pmag) Otherwise, the mags ran fine. very odd.

    5. Looks like you just need a different buffer

    6. My guess is not enough gas pressure to properly cycle the weapon. If you have an adjustable gas block a small tweak should do the job.

  45. Jeremiah,

    That should work just fine. You aren’t going to ruin anything by putting a rifle length upper on you carbine lower.

    1. Why are folks talking about carbine/rifle/mid/etc lowers? There’s no such thing. Lowers do not have anything to do with gas systems that are all contained in the upper.

      1. Technically, you are correct, but my guess is that people are associating the buffer assy. with the lower.

      2. Technically you are correct, but my guess is that people are associating the buffer assembly with the “lower”

      3. Lower receiver extensions do have carbine and rifle lengths for the buffer.

    2. I have a 308 ar with a 20” barrel with adjustable gas block. It shoots
      Hornady and Federal 150 to 168 gr with no problems . I just purchase reaper outdoors 145 gr ammo and it will not cycle. Could this be a bullet problem or may I need to adjust gas block.

      1. Adjust your adjustable gas block

  46. i have a lower receiver with carbine length buffer tube assembly.
    i want to use and upper receiver with a 20″ barrel. My question is, will the carbine buffer assembly cycle properly with a rifle length gas system?

    1. No it will not because the tube is to short and it will not cycle properly. The gas pressure will be to high and may cause a jam because it will cycle to fast with the higher pressure and may also cause bolt carrier damage.

      1. I have 14.5″barrel with the gas port at 5″
        What tube should I get?

      2. So much fail in this post, do not listen to Clifford the dog. Rifle uppers can function fine with complete carbine buffer systems.

    2. Just wondering whether a 9inch handguard would be good for a
      L.W. 6mm arc build for hunting and carrying through woods, I already have gas block titanium, f1 and tube .it should look nice but would 9 inch handguard be to a short or. Just go 10.5 or 11, I’m trying to save ounces. Polymer lower and upper,. Type 2 bolt I assume? I will scoot

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