When changing out parts on your AR 15 upper (for example adding a free float handguard/ quad rail), you may need to remove the front sight base and add a low-profile gas block. Unfortunately, this will often leave you with exposed taper pin holes, and since most manufacturers don't "parkerize" under the FSB, you end up with rings of bare metal. The result is an unsightly exposed area on the barrel that looks something like this:
The best way to fix this issue? Use a longer low profile gas block, that is specifically designed to cover this area and protect and strengthen the barrel in this area.
AT3 Tactical sells a low profile gas block that suits this need perfectly! Check out the AT3 Long Low Profile Gas Block!