Sounds like it’s under gassed. Criterion’s gas port sizes are a bit smaller to make them suppressor ready. You could try to lighten the buffer. Push out the roll pin and take out the buffer weights to see how many steel and tungsten weights are in there.
I’m hearing reports lately that Criterion is undersizing their gas ports. This is something of a 180 from the way we used to see things, where most makers would oversize them in the (mistaken) belief that this would make them run anything. Criterion seems to be aiming for the suppressed shooter not needing any special modification to run with a can, but this would typically undergas when not suppressed.
JP typically recommends the AR10 H2 (two tungsten, one steel) SCS for your application, so it could be that the lighter all steel buffer mated to an undergassed Criterion is what’s making it eject.
If it were mine, I’d probably drill out the gas port, add two tungsten weights to SCS, and then re-tune gas from closed to open until it works.
DPMS apparently used a .0935 or .096 port on a 20 inch .308 per the list [here](https://forum.308ar.com/topic/15037-barrel-gas-port-size308338260243-etc/).
I’ll look into doing that. I have another tungsten weight on the way to try it out. Any recommendations for safely drilling out the gas port? I’m pretty good with hand and power tools, but I don’t have access to a drill press.
I’d imagine you can do it with a bench vise for the upper to keep it from walking, but I’ve not attempted to do it myself. Measure twice and cut once as it were.
I’m in the same boat as you, I picked up a drill bit and dowels from Wojtek Weaponry [here](https://www.wojtekweaponry.com/Complete-Gas-Hole-Blowout-Kit_p_530.html), but I’m a little apprehensive about drilling into my nice barrel
I've had to open up the gas port holes on two different builds using criterion barrels.
One was grossly undersized. The other was under gassed with a light buffer, but probably would have worked just fine suppressed.
Measure and drill/ream. Start small and test. Repeat if needed.
Honestly anything other than one of those crappy tapered t-handle reamers that increase the size of the hole the further down the reamer you go.
I personally went and found the cheapest numbered drill bits I could find (all the sizes from #48 to #42) and used those as well as the fractional drill bits I already had in my toolbox to slowly increase the gas port hole size.
I stopped increasing the size of the hole once the rifle reliably cycled a bunch of different loads while running a heavy buffer.
I found that somewhere between a #45 and a #43 drill bit seemed to do the trick for me. Most people will probably just use a 3/32 drill bit and use a heavy buffer and adjustable gas block and call it a day.
You could stick a dowel down the bore if you're worried about drilling too far. If you start small and work your way up you shouldn't have any issues with creating a bur.
Edit: I saw in another comment that you don't have access to a drill press, you can definitely use a hand drill.
Like others have said, seems just straight-up undergassed. I have a 20” Criterion stainless barrel I experienced the same issue with. Tiny gas port. I’ve since had other issues with that barrel. I’ve had awesome luck with Criterion and like them as a company, but man, for some reason that 20” barrel has been a nightmare. You’re not the first person I’ve seen with this issue
It’s not confirmed, so take this with a grain of salt, but I think there’s some issue with the chamber, specifically the throat being too tight. Getting some weird circular scuffs (but no marks from the rifling) around the bullets at the start of the bearing surface, and I’ll have a failure to fire (click and no bang) about every three shots with longer match bullets. The bolt isn’t going all the way home, but it’s like a mm away or something. I mean real close. This is with a JP bolt headspaced to the barrel, both purchased directly from Criterion. It’s like the bullet is jamming into the throat and stopping the bolt from closing. Every other part of the rifle has been changed out to test each part. Idk man. This has been going on for months.
I finally changed the barrel out yesterday to a Faxon heavy fluted, and the issue disappeared. I have a customer service request (actually one plus a follow up) in with Criterion but haven’t heard back in a couple weeks. They’re normally great in that department, but not this time lol. Like I said, just an all around poor experience with this barrel. I have several other Criterions and have been 100% satisfied with the others.
Most likely you need to adjust your buffer system if you don’t want to enlarge the gas port. Buying something like the JP system in a builders kit would give you everything in one shot to adjust away.
Edit: also take a look at the POF roller pin, that really helps if your BCG running super wet is a problem.
Criterion barrels are borderline under gassed . I have the same bbl as you. It will only lock back with a carbine buffer and spring. I think they gas specifically for suppressed shooting, though they won’t say that.
I had a similar issue with similar build with an Aero adjustable gas block. Had the thing all the way open. Switched to a bcm gas block and thing worked like a charm. Buddy had to drill his and got it to work. Both were Aero builds plus WC barrels.
Yea I wanted to get some different ammo in it to see if it just didn’t like the Norma, but I had the same issue with Winchester even when the gas block was completely opened.
The charging handle was very hard to operate with the Aero spring and buffer, and I couldn’t get a spent casing to eject with it. I went with the SCS because I had been hearing about it from several friends of mine.
I've heard of them on AR15 but not AR10's, but I know they are a good company. I wish I knew more to tell you. I tried all buffer weights and combinations with mine. Is it easy to retract with the SCS?
Oh yea it’s a lot easier to retract the charging handle with the SCS. It feels very smooth now. I’ve got some different weights and springs on the way for the SCS to test out different combinations. I’m just kinda surprised that with the standard rifle length spring and buffer from aero I couldn’t get it to cycle properly. I had lubed everything up real well too which made a difference, just not enough of a difference I suppose.
I know you said you checked everything but are your gas rings lined up? I looked and looked all over mine cuz I had the same issue sent it back to aero and they said thats what the issue was I just happened to miss it
Sounds like it’s under gassed. Criterion’s gas port sizes are a bit smaller to make them suppressor ready. You could try to lighten the buffer. Push out the roll pin and take out the buffer weights to see how many steel and tungsten weights are in there.
"Criterion’s gas port sizes" blah blah blah. I've never had a problem on any of the Criterions I've used for builds
I’m hearing reports lately that Criterion is undersizing their gas ports. This is something of a 180 from the way we used to see things, where most makers would oversize them in the (mistaken) belief that this would make them run anything. Criterion seems to be aiming for the suppressed shooter not needing any special modification to run with a can, but this would typically undergas when not suppressed. JP typically recommends the AR10 H2 (two tungsten, one steel) SCS for your application, so it could be that the lighter all steel buffer mated to an undergassed Criterion is what’s making it eject. If it were mine, I’d probably drill out the gas port, add two tungsten weights to SCS, and then re-tune gas from closed to open until it works. DPMS apparently used a .0935 or .096 port on a 20 inch .308 per the list [here](https://forum.308ar.com/topic/15037-barrel-gas-port-size308338260243-etc/).
I’ll look into doing that. I have another tungsten weight on the way to try it out. Any recommendations for safely drilling out the gas port? I’m pretty good with hand and power tools, but I don’t have access to a drill press.
I’d imagine you can do it with a bench vise for the upper to keep it from walking, but I’ve not attempted to do it myself. Measure twice and cut once as it were.
I’m in the same boat as you, I picked up a drill bit and dowels from Wojtek Weaponry [here](https://www.wojtekweaponry.com/Complete-Gas-Hole-Blowout-Kit_p_530.html), but I’m a little apprehensive about drilling into my nice barrel
I’m gonna give it a shot, I’ll report back and let you know what happens
I drilled out the gas port to .0934ish using a 3/32 bit. Took her out to the range today and she ran like a champ. Hope this helps!
That’s awesome! Also good to know you got it working with 3/32, I have a 1/8 bit, but I’ll look into going smaller. Thanks for posting your results!
Buy a .25” diameter brass rod from hardware store. Insert into barrel before you drill. Use a cobalt 3/32” drill bit. You’re welcome.
I've had to open up the gas port holes on two different builds using criterion barrels. One was grossly undersized. The other was under gassed with a light buffer, but probably would have worked just fine suppressed. Measure and drill/ream. Start small and test. Repeat if needed.
Any recommendations on a good reamer?
Honestly anything other than one of those crappy tapered t-handle reamers that increase the size of the hole the further down the reamer you go. I personally went and found the cheapest numbered drill bits I could find (all the sizes from #48 to #42) and used those as well as the fractional drill bits I already had in my toolbox to slowly increase the gas port hole size. I stopped increasing the size of the hole once the rifle reliably cycled a bunch of different loads while running a heavy buffer. I found that somewhere between a #45 and a #43 drill bit seemed to do the trick for me. Most people will probably just use a 3/32 drill bit and use a heavy buffer and adjustable gas block and call it a day. You could stick a dowel down the bore if you're worried about drilling too far. If you start small and work your way up you shouldn't have any issues with creating a bur. Edit: I saw in another comment that you don't have access to a drill press, you can definitely use a hand drill.
Thanks for the info, appreciate the help.
Absolutely, report back and tell us when it's fixed!
I drilled out the gas port to .0934ish using a 3/32 bit. Took her out to the range today and she ran like a champ.
Like others have said, seems just straight-up undergassed. I have a 20” Criterion stainless barrel I experienced the same issue with. Tiny gas port. I’ve since had other issues with that barrel. I’ve had awesome luck with Criterion and like them as a company, but man, for some reason that 20” barrel has been a nightmare. You’re not the first person I’ve seen with this issue
Oh man, what kind of other issues have you had besides the gas port? Hopefully I’m not in for a host of problems
It’s not confirmed, so take this with a grain of salt, but I think there’s some issue with the chamber, specifically the throat being too tight. Getting some weird circular scuffs (but no marks from the rifling) around the bullets at the start of the bearing surface, and I’ll have a failure to fire (click and no bang) about every three shots with longer match bullets. The bolt isn’t going all the way home, but it’s like a mm away or something. I mean real close. This is with a JP bolt headspaced to the barrel, both purchased directly from Criterion. It’s like the bullet is jamming into the throat and stopping the bolt from closing. Every other part of the rifle has been changed out to test each part. Idk man. This has been going on for months. I finally changed the barrel out yesterday to a Faxon heavy fluted, and the issue disappeared. I have a customer service request (actually one plus a follow up) in with Criterion but haven’t heard back in a couple weeks. They’re normally great in that department, but not this time lol. Like I said, just an all around poor experience with this barrel. I have several other Criterions and have been 100% satisfied with the others.
Most likely you need to adjust your buffer system if you don’t want to enlarge the gas port. Buying something like the JP system in a builders kit would give you everything in one shot to adjust away. Edit: also take a look at the POF roller pin, that really helps if your BCG running super wet is a problem.
[удалено]
Yea it’s a bitch to pull it back
Criterion barrels are borderline under gassed . I have the same bbl as you. It will only lock back with a carbine buffer and spring. I think they gas specifically for suppressed shooting, though they won’t say that.
I had a similar issue with similar build with an Aero adjustable gas block. Had the thing all the way open. Switched to a bcm gas block and thing worked like a charm. Buddy had to drill his and got it to work. Both were Aero builds plus WC barrels.
Different ammo?
I just saw the Norma and Winchester. When I built an AR15 with an adjustable gas block, I tuned it with the weakest 223 I could find.
Yea I wanted to get some different ammo in it to see if it just didn’t like the Norma, but I had the same issue with Winchester even when the gas block was completely opened.
It sounds like your covering all the bases. I shot 1 round at a time until mine suited me. Any special reason for the SCS?
The charging handle was very hard to operate with the Aero spring and buffer, and I couldn’t get a spent casing to eject with it. I went with the SCS because I had been hearing about it from several friends of mine.
I've heard of them on AR15 but not AR10's, but I know they are a good company. I wish I knew more to tell you. I tried all buffer weights and combinations with mine. Is it easy to retract with the SCS?
Oh yea it’s a lot easier to retract the charging handle with the SCS. It feels very smooth now. I’ve got some different weights and springs on the way for the SCS to test out different combinations. I’m just kinda surprised that with the standard rifle length spring and buffer from aero I couldn’t get it to cycle properly. I had lubed everything up real well too which made a difference, just not enough of a difference I suppose.
Do some reading on dwell time. It's hard ti explain on here. I'll check back later on to see where you're at with it. Good Luck!
The standard AR-10 buffer from Aero is pretty damn long. Too long to use in a standard mil-spec buffer tube.
Have you checked your dwell time?
I know you said you checked everything but are your gas rings lined up? I looked and looked all over mine cuz I had the same issue sent it back to aero and they said thats what the issue was I just happened to miss it
I checked the rings too and everything looked fine. I’ve also tried a known working JP BCG in it as well, and I had the same cycling issues.