AR-15 Bolt Carrier Group Rebuild

Years ago I purchased a PSA bolt carrier group that gave me nothing but trouble from the start. I put it away and didn’t use it for a while, but a few months ago I decided that I wanted to get it working again, and I’ve finally gotten around to it. In the interest of gaining experience (and content for the blog), I chose to do a complete rebuild of the bolt carrier group.

The subject bolt carrier group

Rebuilding a BCG is remarkably easy as long as you have a few simple tools:

  • A 1/8″ Allen bit
  • A socket wrench
  • An inch/lb torque wrench
  • A staking tool
  • A bolt disassembly tool

The first thing to do is field strip your BCG:

Bolt Carrier

I decided to rebuild the carrier first, since there’s only one piece to work on – the carrier key. I placed it in a vice (with padded jaws to avoid damage) and removed the key screws. I originally tried removing them with an Allen key, but the staking and torquing was good enough that I ended up needing to use a socket wrench to break them free.

Carrier with carrier key removed

Once I had the key removed, I cleaned underneath it to ensure the surface of the carrier would mate cleanly with the key. I placed a small amount of liquid gasket around the gas port to ensure it sealed correctly, then placed the new key on the carrier.

Important note: the military technical manual and Colt armorer manual differ on the necessity of the gasket. The Colt manual states that it should be used, while the military manual states that a field-expedient method is firing 3-5 blank rounds to create a carbon seal underneath the key. I chose to use the Colt method.

I placed the key screws in their spots, torqued to spec, and then used the Brownells Carrier Key Staking Tool to stake. The torque spec on the screws is 50-58 in/lbs. I torqued to 54 in/lbs to keep it right in the middle.


Next, I completely stripped the bolt using a 1/16″ punch and the Sinclair Bolt Vise.

Once it was stripped, I cleaned it and started reassembling. I went a little out of spec here and decided to use a helical (or McFarland) gas ring instead of the standard 3 rings. The McFarland gas ring was used in some of the initial Mk18 builds, and is thought to be a little more durable than the standard 3 rings. A lot of that seems to be personal preference, though, and I wanted to try one out.

Next, I installed the new ejector, ejector spring, and ejector pin with the bolt vise. The ejector pin is considered an expendable item – once you remove it, you need to throw it away and install a new one.

After the ejector was installed, I assembled the new extractor and added the O-ring, which the previous extractor assembly did not contain, then installed it on the bolt.

Finally, I reinstalled the bolt into the carrier and conducted a quick test to ensure the gas rings were properly sealing inside the carrier body. Now I just need to get to the range and test fire…


As I said at the beginning, rebuilding a BCG is pretty straightforward. It only took me about an hour of actual work, although it was slowed greatly by stopping to take pictures. You won’t have to do it often (you may never need to do it), but it’s good to know how to do it if it comes up.

I hope this has been useful for you. If you enjoyed, give us a follow and a share. Social media really limits our ability to advertise, and organic shares always have the biggest impact. If you think I missed something or have a question, drop a comment below.

Thanks for reading!

Leave a Reply