My Criterion (18", 1:10, .308) came with a .068 rifle length gas port. I could never get the bolt to hold open on heavy loads, and several brands of 150gr FMJ stuff wouldn't cycle...period...and I spent a lot of time and money on parts trying to remedy the problem.
After reading up on .308 gas port sizes, everything I found pointed to my gas port being undersized for that barrel length. I contacted Criterion, and was politely told that I was wrong and so were the websites.
Frustrated, I didn't know what to do until a nice gentleman from here who makes AR barrels PM'ed me and told me to open the port to .093 with my drill press. I guess he'd made a few thousand with the same barrel and gas length, and had always gone .093 (which is what I found on the internet).
I took my time with my press, and wow...never had a short stroke since. Took that rifle on quite a few hog hunts and it fed everything great without any other change. It would be a touch overgassed if you ran a non-adjustable gas block...but an AGB is an easy thing to tune.
I had that upper apart completely three times trying to find out where I was wrong. I'd also spent a lot of money on springs and buffers by that point...guess I have more spare parts on the bright side.
That 18" Criterion shoots well too...but I'll never buy another of their barrels.
Just my mileage, YMMV.
