Function problem with a .308 build:

[Sgt Dog]

The symptoms were common ones: Fired empty cases dribbling out, and failing to pick up and feed the next round, failing to lock back on an empty magazine. So I dove into some AR specific forums with questions.

After reviewing my choices for spring and buffers and airing both the symptoms and back-end set-up, posters convinced me it was in the front end. Stripped it down to confirm gas block port alignment, obstructions, etc.

After finding nothing there I was asked this question: “Did you ever check the diameter of the gas port in the barrel? Your recoil system is good, which leaves the gas system as the culprit”.

Dang!! A guy WOULD have already put the block, tube and hand-guard back on. So I repeat the tear-down and sure enough, the port is about 50% (by volume undersized) at .0770. I called Michael at Criterion Barrels to confirm. He couldn’t believe they shipped like that. Then he offers a shipping label and wants the whole upper shipped.

Meanwhile I reluctantly let myself be convinced that re-drilling the gas port myself is a piece of cake. (And btw, Criterion never followed-thru on the offer of a shipping label).

Anyway, the forum posters were right. Once I got the bit aligned with a tower of boxes holding my vice block, vice and rifle in position it WAS a piece of cake. I used a brass rod in the barrel to make sure I did no damage.

Anyway, lesson learned. And some experience gained. “Check the port size next time before you assume”! Especially with a .308.

In the short video I should have lingered on the stacked boxes/vice/rifle required to get the right alignment & height… if for nobothervreason than to show another budda at work. Kinda like the guys you see on a ladder set up on a stool off a lumber rack on a pickup bed!

 

[diggler1833]

Man...I had the same stupid problem with a Criterion in a .308 build.

I went so far as to strip my upper down three times (once was because I had an obstructed CMC nitride gas tube...never again). I couldn't get it to work worth a darn unless I was at a max charge with a 175+ gr bullet.

I measured my gas port in my 18" Criterion, and it was .068". One of the .308 specific forums had that listed as needing to be .093" for my tube length. I called Criterion and was told that the .068 was correct and I was screwed up somewhere else.

Two more buffers later and I was back on the forums. The owner of AR Performance messaged me and told me to use a drill to open it up to .093 as he'd done a "few thousand" that way. Sure enough as soon as I ordered a .093 HSS bit, chucked it in my press, and drilled the hole out...I’ve yet to ever have a problem with that rifle since.

The good news is that I became a gas port/block/tube alignment champion. The bad news is that I'll never trust another Criterion barrel.

 

[Seadog]

Man...I had the same stupid problem with a Criterion in a .308 build.

I went so far as to strip my upper down three times (once was because I had an obstructed CMC nitride gas tube...never again). I couldn't get it to work worth a darn unless I was at a max charge with a 175+ gr bullet.

I measured my gas port in my 18" Criterion, and it was .068". One of the .308 specific forums had that listed as needing to be .093" for my tube length. I called Criterion and was told that the .068 was correct and I was screwed up somewhere else.

Two more buffers later and I was back on the forums. The owner of AR Performance messaged me and told me to use a drill to open it up to .093 as he'd done a "few thousand" that way. Sure enough as soon as I ordered a .093 HSS bit, chucked it in my press, and drilled the hole out...I’ve yet to ever have a problem with that rifle since.

The good news is that I became a gas port/block/tube alignment champion. The bad news is that I'll never trust another Criterion barrel.

I had a similar occurrence. I built a MK12 mod 0 using a Criterion barrel in 5.56. The rifle was short stroking. Sometimes it would work but mostly it wouldn’t. It would fire off a round and then click, nothing.

The gun was so under gassed it would throw the empty brass out but it won’t go far enough back to strip a new round out and load it. No jamming. Just not going far enough back. After researching the gasport size I realized mine was undersized from what it should be. So me three, with the gas port being undersized. Bought the correct drill and opened it up. Now the rifle works. Other than the undersized gas port it has been an extremely accurate rifle.

 

[diggler1833]

I had a similar occurrence. I built a MK12 mod 0 using a Criterion barrel in 5.56. The rifle was short stroking. Sometimes it would work but mostly short stroke. After researching the Gasport size I realized mine was undersized from what it should be. So me three, with the gas port being undersized. Bought the correct drill and opened it up. Now the rifle works. Other than the undersized gas port it has been an extremely accurate rifle.

Yep. My barrel is pretty accurate too...definitely accurate enough that I would have recommended them otherwise. However that isn't happening after my experience.

 

[Sgt Dog]

Thanks for adding to this fellas!

Michael at Criterion was as good as gold on the phone. Not so much in follow-thru! Said he was sending me a shipping label and he’d get in it. Two messages and several weeks later and I’ve not heard a peep from Michael at Criterion!!

Btw, I suspect 0.093 may be the optimum. At 0.096 I had to tune the adjustable gas block down considerably.

 

[Seadog]

Thanks for adding to this fellas!

Michael at Criterion was as good as gold on the phone. Not so much in follow-thru! Said he was sending me a shipping label and he’d get in it. Two messages and several weeks later and I’ve not heard a peep from Michael at Criterion!!

Btw, I suspect 0.093 may be the optimum. At 0.096 I had to tune the adjustable gas block down considerably.

What drillbit size is .096 and where did you get it

 

[diggler1833]

Thanks for adding to this fellas!

Michael at Criterion was as good as gold on the phone. Not so much in follow-thru! Said he was sending me a shipping label and he’d get in it. Two messages and several weeks later and I’ve not heard a peep from Michael at Criterion!!

Btw, I suspect 0.093 may be the optimum. At 0.096 I had to tune the adjustable gas block down considerably.

.093 works exceptionally well with 147-150gr bulk/surplus stuff. I bet you could get away with factory match/hunting ammo and .088...but why risk it if you have an adjustable gas block? Either way, it sounds like yours is running well and that is a good thing.

 

[Sgt Dog]

What drillbit size is .096 and where did you get it

The note I left myself in the drawer is #41. I had to drive to a hardware store downtown after calling around. I’d send you one if you don’t have any luck.

I do wonder about the 0.093 vs the 0.0960 though. I’m functioning but turned down by half on the gas block shooting 168gr Federal Gold Dot. When I say ‘half’ I mean nearly half in ‘clicks’. That could be deceiving. I’ll look for a thread I started in which fellas chimed into that has a boatload of info. The claims were 0.0960 was the call-out for an 18” barrel. Changes significantly with length. But I wonder. And volume calculations change significantly as you increase in size.

I’ll post a link to the thread if I find it. Its worth the read for a couple imbedded links pointing back to the .308Community forum. One discusses the length if the gas tube and how you can determin if that could be an issue. It frequently is, because a lot of suppliers provide one 3/16” too short. This one is too long. Should fall exactly in the middle of that cut-out. To short is worse than too long - for reasons of escaping gas in the gas key.

 

[Sgt Dog]

Look at page two of this thread for a post by dpete with two embedded links in it. Those two links have a boatload of info. First is on port size. Second is on gas tube lengths.

AERO .308 Short Stroke Specifics & Symptoms: - Page 2

AERO Short Stroke Specifics & Symptoms: *Fails to consistently strip ammo from magazine. *Fails to lock back on empty magazine. *Ejected cases barely clear shooting bench. Factory (quality) 168gr ammo. Superlative Adjustable gas block (wide open) 18 barrel with rifle length gas port...

www.m4carbine.net

 

[Seadog]

The note I left myself in the drawer is #41. I had to drive to a hardware store downtown after calling around. I’d send you one if you don’t have any luck.

I do wonder about the 0.093 vs the 0.0960 though. I’m functioning but turned down by half on the gas block shooting 168gr Federal Gold Dot. When I say ‘half’ I mean nearly half in ‘clicks’. That could be deceiving. I’ll look for a thread I started in which fellas chimed into that has a boatload of info. The claims were 0.0960 was the call-out for an 18” barrel. Changes significantly with length. But I wonder. And volume calculations change significantly as you increase in size.

I’ll post a link to the thread if I find it. Its worth the read for a couple imbedded links pointing back to the .308Community forum. One discusses the length if the gas tube and how you can determin if that could be an issue. It frequently is, because a lot of suppliers provide one 3/16” too short. This one is too long. Should fall exactly in the middle of that cut-out. To short is worse than too long - for reasons of escaping gas in the gas key.

Thanks for that answer on the #41 drill bit. My lazy butt did a little bit of research and already figured that out. I ordered a few carbide drill bits online. A #41 and a #42

I assembled a rifle that I haven’t tested out yet. I figure when the drill bit comes in if it goes in it’s the right size if it doesn’t then it’s undersized. I have a 20” ballistic advantage top of their line stainless steel barrel that is bead blasted a nice dull gray. I finally got a few parts in for it this last week and finished it up. I need to go out and function test it but I figure I would get a drill bit to check the gas port hole size 1st before wasting ammo.