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Building an AR-15 and AR-10. What Parts are Compatible?

When Eugene Stoner and the team from Armalite began work on the AR-10, they wanted a rifle that was serviceable in the field, even when there wasn’t a gunsmith on hand to fix problems and fit parts. In order to achieve this serviceability, the gun had to be fully modular. This means parts from one AR-10 (and the AR-15, which would come shortly after the AR-10) should–in theory at least–fit on any other AR-10 or AR-15.

The AR-15 is built to be modular and its parts should work on any other AR-15. The same is not true for the AR-10.
The AR-15 is built to be modular and its parts should work on any other AR-15. The same is not true for the AR-10.

For the most part, these guns have stayed true to this original specification. Parts are easily swapped out of AR platform rifles and pistols. There are some oddballs that look like the rest of the family, but really aren’t–but that’s not the subject for today’s post.

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The aim here is to talk about a different kind of modularity. We know it is easy enough to swap out parts between AR-15s, but what about between the AR-10 and the AR-15?

The AR-10 has two predominant designs for uppers. You'll need to know if you have an Armalite design or a DPMS pattern to your receivers.
The AR-10 has two predominant designs for uppers. You’ll need to know if you have an Armalite design or a DPMS pattern to your receivers. Either way, the AR-10 fires a larger round, so some parts are sized accordingly.

What is an AR-10?

The AR-10 began life as a 7.62 x 51. This was the round that the M-14 fired, and the Army liked the fat .30 caliber bullet and its stopping power at longer distances. But those cartridges are heavy. And the AR-10 (though lighter than most M-14 variants) isn’t a flyweight.

With a few modifications, the AR-10could be sized down for the 5.56. This became the AR-15, a rifle that was lighter and smaller, even though it shared some of the AR-10’s parts.

The Upper of an AR-10 is slightly larger than that of the AR-15, so there is no comparability between these parts.
The Upper of an AR-10 is slightly larger than that of the AR-15, so there is no comparability between these parts.

Which parts aren’t compatible?

Let’s start with the big ones. Because these guns fire different rounds, the barrels aren’t compatible. This would seem pretty obvious to most of us.

The lower receivers aren’t compatible, either. The 5.56 is shorter. While the .308 is considered one of the short-action calibers, it isn’t a super short round and the AR-15 lower’s mag well is sized for the 5.56 and not for all short-action calibers (even when they are stamped as multi-caliber).

AR-10’s have a longer lower, which requires a longer upper to match. And the bolt is longer and has a wider bolt-face. And the buffer is typically shorter.

At the very least, these elements will stay dedicated to their respective platforms. And, because some of these pieces are larger, other parts are affected.

The mag wells on the AR-10 and AR-15 reflect the distinct size of the cartridges that have to feed from them.
The mag wells on the AR-10 and AR-15 reflect the distinct size of the cartridges that have to feed from them.

Think about the design

Think about an AR in its basic shape. Cartridges are fed in, chambered, fired, and extracted. Almost all of the parts that come in contact with that cartridge are size specific. From the magazine to the bolt-face to the muzzle device, they all have to be equipped for the round or cartridge of one given platform.

Now think about the parts of the gun that don’t touch the round. This includes furniture, sights, some controls, some internal parts…. It would be most efficient to design these to work on either rifle. And that’s pretty much what we have.

Magazines for both are not interchangeable. The AR-15 is much more compact.
Magazines for both are not interchangeable. The AR-15 is much more compact.

Reliably compatible parts between the AR-10 and AR-15.

When it comes to these parts, you can typically count on them working. Typically. There’s always an exception to the rule.

The uppers of the two guns have to accommodate the respective length of their bolt-carriers (and the .308 is a bit longer).
The uppers of the two guns have to accommodate the respective length of their bolt-carriers (and the .308 is a bit longer).

Why this gets complicated

The vast majority of AR-15s can accept parts from other manufacturers, the AR-10 is different. There is one basic set of measurements for the AR-15, and manufacturers may make many cosmetic changes, but they’ll reliably leave the important dimensions alone.

The AR-10 never took off. It didn’t enter into widespread military contract production. The design built a loyal following, but manufacturers have felt more liberty in making substantial changes to the platform that make these designs proprietary.

The angled corner on the AR-10 is easy to spot and will will require an angled cut on the upper.
The angled corner on the AR-10 is easy to spot and will will require an angled cut on the upper.

There are two dominant design variants for the AR-10, though, that have stuck around. Some companies stick pretty close to the old Armalite design. Other companies have followed the lead of DPMS (also known as the LR-308).

The LR-308 (or sometimes called a DPMS lower) has a curve, just like the AR-15.
The LR-308–this one by Aero Precision–is sometimes called a DPMS lower. The pattern has a curve, just like the AR-15.

The AR-10 has a very angular slope to the buffer extension. The DPMS has a radiused curve that mirrors the curve on the typical AR-15 lower.

Many of the parts that connect to the lower, though, are interchangeable. Grips work, and stocks.
Many of the parts that connect to the lower, though, are interchangeable. Grips work, and stocks.

These two dominant types are easy enough to tell apart, once you know what you’re looking for. Just check that corner on either the upper or the lower. Knowing which parts will work with each lower, though–that’s a different issue.

Why bother with AR-10 and AR-15 compatibility?

If you’re made of money, odds are you won’t have to bother. But maybe you’ve got that DIY itch. You want to build the perfect rifle, and you need to know.

An AR-10's upper is longer, so an AR-15 charging handle isn't long enough to work.
An AR-10’s upper is longer, so an AR-15 charging handle isn’t long enough to work.

Or, if you’re like a lot of AT3 devotees, you already have some spare parts kicking around. These parts tend to build up quick for some of us. We’re always tinkering, moving things around, swapping parts out, and there’s no good reason why any of those shouldn’t have a home. Knowing what will fit on one platform or the other can help you put together a brand new rifle–and maybe one in a different caliber.

The real dilemma will come when you want to get into the oddballs and the wildcats. The .308 is, in most bolt-action rifles, considered a short action. Step up you AR game to the .30-06, or anything larger and you’ll run into a select group of rifle manufacturers (like Noreen) that build even more boutique shapes and sizes.

If you’re new to building, start with an AR-15. You’ll graduate to the AR-10 in good time. And then you’ll need to fill in the gaps. 300 BLK. .22 LR. .450 Bushmaster…. The safe will likely get a bit crowded, but there’s nothing wrong with that.

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.

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