13.7" build. Need help troubleshooting please!

[Buffman]

XM855/M855/SS109 are all essentially the same bullet. The difference is who's loads are you using? What brand M193, M855, and the SS109 you mentioned?

[Moleman-]

You mentioned that you can't remove the gas block because of the pinned muzzle device, but can you slip it forward the 3/8" to see if the carbon staining around the gas port is even, and measure the gas port. Drill bits usually drill a larger hole than what the shaft diameter is, and the shaft diameter is rarely the same size as the number on the bit. So if you have a big 120+ drill bit index see what's the biggest bit shaft that'll fit in the port and measure the bit unless you happen to have a .061"-.250" pin gauge set.

[AleksanderSuave]

If a different BCG doesnt resolve it, then I would think its not exclusively the BCG, all though both might have issues. the buffer system you have normally solves a lot of issues that others wont. I'd be curious to see how it runs with a regular buffer as opposed to the vltor A5.

As others have said, the gas port sounds like it could be the problem. Normally without the pinned muzzle device you could at least try to resolve it with an adjustable gas block (this completely solved my BA short barrel build's problems, even with their own BA low profile block being somewhat unreliable).

I guess you have to decide if you want to break the weld on the MD to test this theory, or send it back? Either way you're either without rifle for a while, or buying a new gas block and paying for a new pin and weld for the MD after.