I’ve seen bad trigger pin hole placement cause this problem.
Gunsmithing AR issue need some advice
- Thread starter Altoonabcm157
- Start date
While your waiting for those gauges to arrive you can use your number drill set , by hand, to find the aprox diameter of your gass port hole.
Take a look at the gas tube, only takes a minute. It should end half way of the cam pin cutout in the upper receiver. If it doesn't end halfway of the cutout then it's too short or too long. Too short will cause the gun to be undergassed even with the proper size gas port. see attached pic for clarification. 
Sorry for the necro-post. I'm having the same problem with my Criterion SS .308 22" hybrid barrel. My gas port size looks to be .080. Should I shoot for +.005 or just jump right to .093?
I was once given the following advice from the owner of AR Performance (on here):
"I've done thousands of (your) barrels at .093", and I've never had one call back about being undergassed."
I went from not cycling anything surplus, to the rifle running like a top. Although I would caveat to suggest adding an adjustable gas block. I'd predict the rifle being a bit over gassed with 168gr+ loads near max pressure.
My Criterion is 18", rifle length gas, .308, but had a .068" gas port which they told me was correct when I called. It ran heavy 180gr loads, but failed to cycle with anything less.
Thanks. Mine would not run with the Federal 185gr match ammo either. It has an SLA gas block already so I might as well use it.
If you drill it yourself go to hardware store and get a wooden Dowell that fits in your barrel to protect the rifling.
i ran into this for the first time myself last week. no worries, it gave me something to do.
first off, tried different magazines. same shit. so i started going through pieces one by one. swapping out pieces resulted in the same shit. finally, i went with my original thought. and my thought was that it seemed like the magazine wasnt riding high enough in the weapon for it to pick up the next round. chambering the first round was fine, but when fired, nada. so i swapped out the entire lower with a different rifle. volia! the lower reciever was the culprit. slapped it on the other upper, it ran like a song. so i ended up just swapping the lowers on the two rifles.
after looking at the obvious stuff, and before you start digging into the no-so-easy stuff...swap that lower out and see what happens.
first off, tried different magazines. same shit. so i started going through pieces one by one. swapping out pieces resulted in the same shit. finally, i went with my original thought. and my thought was that it seemed like the magazine wasnt riding high enough in the weapon for it to pick up the next round. chambering the first round was fine, but when fired, nada. so i swapped out the entire lower with a different rifle. volia! the lower reciever was the culprit. slapped it on the other upper, it ran like a song. so i ended up just swapping the lowers on the two rifles.
after looking at the obvious stuff, and before you start digging into the no-so-easy stuff...swap that lower out and see what happens.
Update. My .085 and .093 reamers showed up. I just happened to measure the .085 reamer shank and it was .079 so I dropped that end into the gas port, which is .080. It dropped in with an almost perfect fit but would not go all the way into the barrel! I either had a nasty burr or the port was not drilled all the way through. I put considerable pressure on the reamer and whatever was in the way was not moving.
Either way I had a restriction of some kind so I reamed the port with the .085.
I'm confident this will solve my problems but what a pain to diagnose.
Either way I had a restriction of some kind so I reamed the port with the .085.
I'm confident this will solve my problems but what a pain to diagnose.
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