SME Help needed; Gas Port Size

[Sparky5019]

Situation:

I built a customer a perfect 10.3” Geissele upper group that was perfect (parts listed below).

Geissele upper
Geissele 10.3 barrel (0.069” port)
SLR sentry 7 AGB taper pinned
Geissele BCG w/ JP gas ring
Geissele MK8 handguard
Geissele ABCH
Surefire SOCOM Closed tine MD
Surefire SOCOM556-RC2

He does use a JP SCS in the lower. More on that later.

Upper runs flawlessly suppressed/unsupressed with any ammo (pending slight adjustment to the AGB). It runs flawlessly for 1000s of rounds. He wants the shoulder on the barrel set back to move the can closer to the rail and make the gun slightly more compact. He takes the gun to a LGS hack (without calling me to ask an opinion or to help him), and the guy proceeds to f*ck up not just my work but make the upper not lock back with any ammo under any conditions. This guy even punched my taper pin all the way through from the WRONG side (put in just like FSB pins). Can ya tell I’m a little pissed? lol It’s a damn miracle the new shoulder was in the right place and actually straight but somehow it was. The muzzle crown and threads were jacked up and I had ADCO fix that (since I don’t have a lathe).

It looks like he cut 3/8” or less from the barrel’s OAL. I wouldn’t think that little would put us in this position but here we are…


The customer calls me up and says hey my gun stopped running. He brings it by and then I find out what happened. Long story short, this guy is a friend so I’m “stuck” fixing this mess.


So gun was ejecting way over gassed with the JP SCS and still not locking back so I dropped the upper on one of my lowers with Geissele Super 42 spring and H2 buffer and it worked better. It locked back occasionally and the ejection was about 4:00 so the bolt carrier speed definitely was slowed.


So customer wants to stay with the S42H2. Now we have to give the system more gas to lock back and be reliable. The question is how far we need to go.


I have gone up to 0.78” incrementally and still occasionally not locking back but ejection is moving forward to around 3:00 so the BCG is picking up some speed. All of this has been tested with the AGB fully “open” and unrestricted.

Is there a max for a barrel this short?


I had an old DD 10.3 barrel that had an 0.080” port so I didn’t want to go too much past that but…


SME help is greatly appreciated! Thanks in advance.

[HKGuns]

Carbine or Midlength gas system? I don't want to just assume Carbine. Also, final length of the barrel is 10"?

I'm not an SME, but this sheet may provide some ideas, until an SME can pop in.....

Documented Gas Port sizes

[Sparky5019]

Carbine gas system. I’ll check out that link. Thanks.

Looks like the shorter the gas system goes relative to barrel length the ports get significantly larger so that helps.

Anyone with experience. Please chime in here!

[Jim D]

10.3" is on the razors edge of reliability in the first place. An adjustable gas block with never be as efficient as a fixed block as any adjustable mechanism can only ever hope to create a seal as good at something without an adjustable valve on it.

Sounds like he learned a valuable lesson to not alter a gun on the edge of reliability for the cosmetics of "less rail gap".

10.3"/10.5" dwell time is minimal in the first place, trying to chase reliability below that in barrel length is a waste of time if you ask me.

Go back to a .070"+/- a hair port size on a 10.3/10.5" barrel, a good sealing fixed gas block, and let the gun do what we know they can with known dimensions/ values. If the dude wants to argue about it, let him take it back to the guy who agreed to chop it down in the first place and let him deal with that mess.

[Sparky5019]

10.3" is on the razors edge of reliability in the first place. An adjustable gas block with never be as efficient as a fixed block as any adjustable mechanism can only ever hope to create a seal as good at something without an adjustable valve on it.

Sounds like he learned a valuable lesson to not alter a gun on the edge of reliability for the cosmetics of "less rail gap".

10.3"/10.5" dwell time is minimal in the first place, trying to chase reliability below that in barrel length is a waste of time if you ask me.

Go back to a .070"+/- a hair port size on a 10.3/10.5" barrel, a good sealing fixed gas block, and let the gun do what we know they can with known dimensions/ values. If the dude wants to argue about it, let him take it back to the guy who agreed to chop it down in the first place and let him deal with that mess.

While I don’t disagree with you, I’m telling a friend to basically f*ck off and that type advice is not what I’m here to illicit. I do understand what optimal specs are…we are beyond that in this situation. Thank you for your opinion.

This thread is here so I can gain some insight on our SME’s EXPERIENCE and KNOWLEDGE of what is possible outside the optimal specs. I would appreciate as objective information as possible.

This is a shitty situation but not nearly as bad as a lot of the dumb shit I see guys post about they want to do to their own guns; cosmetic or not.

Thanks in advance for the help.

[Jim D]

While I don’t disagree with you, I’m telling a friend to basically f*ck off and that type advice is not what I’m here to illicit. I do understand what optimal specs are…we are beyond that in this situation. Thank you for your opinion.

This thread is here so I can gain some insight on our SME’s EXPERIENCE and KNOWLEDGE of what is possible outside the optimal specs. I would appreciate as objective information as possible.

This is a shitty situation but not nearly as bad as a lot of the dumb shit I see guys post about they want to do to their own guns; cosmetic or not.

Thanks in advance for the help.

What do you think makes someone an SME on this topic?

[Sparky5019]

What do you think makes someone an SME on this topic?

The admins have given them a profile title in yellow of “Subject Matter Expert”.

I respect that you have extensive knowledge given what you do for a living but I’m sorry, I will not be telling a friend to f*ck off buy a new upper because you did not ask questions. He learned from this experience and fixing his issue will solidify him as a future customer. I’m sure y’all have done the same thing.

[Defaultmp3]

It looks like he cut 3/8” or less from the barrel’s OAL. I wouldn’t think that little would put us in this position but here we are…

Perhaps the Geissele barrel had a swage, like some CHF barrels, and this was contributing to the gas drive, and with the chop, it was reduced, beyond just the issue of the further reduced dwell time.

So gun was ejecting way over gassed with the JP SCS and still not locking back so I dropped the upper on one of my lowers with Geissele Super 42 spring and H2 buffer and it worked better. It locked back occasionally and the ejection was about 4:00 so the bolt carrier speed definitely was slowed.

Did you mean undergassed?

Is there a max for a barrel this short?

I had an old DD 10.3 barrel that had an 0.080” port so I didn’t want to go too much past that but…

I don't see why you wouldn't. Gas port size is just one of many issues at play, there is also the size of the bore itself (I recall Amphibian over on ARFCOM doing some testing back in the day show differences in gas port sizing between two different barrels from the same manufacturer in order to get optimized performance with the exact same suppressor), ammunition, lubrication, etc.

The admins have given them a profile title in yellow of “Subject Matter Expert”.

I respect that you have extensive knowledge given what you do for a living but I’m sorry, I will not be telling a friend to f*ck off buy a new upper because you did not ask questions. He learned from this experience and fixing his issue will solidify him as a future customer. I’m sure y’all have done the same thing.

Someone with the title Subject Matter Expert title here doesn't mean they're a SME on ARs. Pretty sure it really just means that they are/were verified SOF.

[Sparky5019]

No it was over gassed. BCG moving super fast and brass going forward but would still not lock back with the JP SCS.

Point about SMEs. I just want solid objective info that doesn’t involve telling a friend to f*ck off and judge the guy for making a mistake. I’m here to fix the problem for him and help him learn from it.

Thanks!!

[Defaultmp3]

No it was over gassed. BCG moving super fast and brass going forward but would still not lock back with the JP SCS.

I'm confused, what combination exactly is overgassed, then? The structure of you post implies that the rifle was overgassed with the JP SCS prior to you opening up the gas port; what was the gas port size when you decided that it was overgassed with the JP SCS?