I built a new upper and shot it today.  The thing does not even try to cycle.  It unlocks the bolt and that's it.  I noticed when I assemble the gas block the gas port seems extremely small.  Can anyone tell me what the right size gas port would be for a 20" rifle system.  Thanks.  Nick

If the barrel is .625" under the gas block, then the gas port should be between .086" and .093".

If the barrel is .750" under the gas block, then the gas port should be between .093" and .096".

I doubt that this is a case of too small a gas port, as you state that there is no bolt movement at all.

Are you using an aftermarket gas block or a standard front sight base? It sounds like you may not have the hole in the gas block/FSB aligned correctly with the gas port in the barrel. Also, check to make sure that the gas tube is installed correctly with the gas hole facing downward.

The barrel is .936" under the gas block.  I'm using an Olympic gas block on an Olympic SUM barrel.  The alignment is good and there is enough pressure to unlock the bolt.  I tried a different BCG and double checked the gas tube and block for flow.  I am sure the port is too small at only .065" but I'm not sure how large it really needs to be.  Hence why I ask.

I have no gas port specs for a barrel that large in diameter, but if you look at the other ones listed above, the larger diameter the barrel, the larger the gas port. .065" definitely sounds small. Hopefully someone else with chime in with better info.

0.065 is smallish even for a carbine gas system. I'd start at 0.105/0.110 for rifle length. barrel diameter has nothing to do with port size, its distance from the muzzle and bore size. This is a 5.56 barrel correct?