Fixed: 18" Alexander Arms barrel: 2.5+ MOA

[LRRPF52]

Piston set-up and an untorqued barrel from the extension are where you need to look before even worrying about accuracy.

Any time you're torquing on a muzzle device, the barrel needs to be isolated, not clamping force on the upper, which is a separate part.

I wouldn't have high expectations from an external op-rod gun in accuracy, unless the gas system was pinned and the barrel bedded really well.

Short story is that there are major issues with a lot of the components and assembly methods you used, before we can even talk about the barrel's accuracy potential.

Another thing with Grendel AR15s is that you have to shoot them differently than .223 ARs in most cases, since the projectile weight and recoil make the gun handle a bit differently as to positional shift.