So. I purchased a 16" MCX barrel off of a friend and have since cut it down to 10.5. It functions perfect with standard ammo, but it will not function properly with subsonic ammunition. The fired round will eject, but it appears as though the bolt does not come back enough to pick up the next round in the magazine. Thus turning my AR into an expensive bolt action rifle. Does anyone have a suggestion on how to correct this issue. I am considering that the gas port needs to be opened, but I cannot find any information about how bid the port needs to be opened to.

Thanks for any assistance.

What caliber is the gun?

Assuming this is a 300 BLK gun.

I don't have both barrel assemblies handy, but I'm pretty sure the gas port on the 16" 300 BLK barrel is considerably farther forward than on my 9" barrel. I think the pistons are also different lengths.

I'm no gunsmith, but I'm not sure if just opening the port would work if the whole gas system is tuned for a length.

The gas system is barrel length specific. The system for one length may not work properly for a different length. Especially longer to shorter.