6mm ARC

[Gtscotty]

Yep good job explaining it.

Tells me the reason the military unit went with longer gas lengths is to better work the gas pressure because they don't run adjustable gas.

My thinking is normal gas lengths will work just fine if one was to use a adjustable gas block.

BFT if your going to go with normal gas lengths, I might just go ahead and get one barrel just for test since all my ar grendels run on lowmass and adjustable gas systems, I could strio one down and throw the 6mm ARC barrel on it and see how it does with a normal gas length but useing adjustable gas.

I think you still want to go with a longer gas system if possible. It's not 100% port size, even with an adjustable gas block, you can still have timing issues that make the gun more finicky to tune. I'm betting that Proof has this this cartridge pretty well figured out, so whatever barrel I buy is going to have the same length gas system, which from their page is rifle for the 16", rifle +1 for the 18", and I'd assume you'd want to go rifle +2 for over 20".