SME Help needed; Gas Port Size

[Sparky5019]

It was over gassed with the JP SCS when suppressed especially. At that point the port had not been altered. The barrel had been shortened at that point.

Once I got the S42H2 in it, then it began to act more “normal” and be under gassed (unsuppressed) which would be consistent with shortening the swell time by cutting the barrel.

We know it needs more gas, the million dollar question is how much more. Lol

[Jim D]

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!!

Well, I literally just spent the last two weekends teaching our Armorers Course to students, and wrote most of the slides we use on dwell time and port sizing…

I don’t know why you seem to think that telling someone that they’ve made a mistake that can not be reliably corrected is telling them to piss off. Part of being trusted in your field is knowing when to tell people no to a bad idea.

It’s highly unlikely that you’ll find a way to make this reduced length barrel reliable now, and simply replacing the barrel and gas block with known quantity items is likely the most cost effective, time effective, and reliable resolution to your “friends” woes. If 5.56mm NATO carbine gas length barrels were viable below 10.3”, I think you’d be seeing at least a couple of them floating around. That you don’t is a clue. You’ll be in experimental territory trying to force this thing to run, with what will likely be an extremely narrow window of circumstances under which it’ll actually work.

Alternatively you could fix this problem tomorrow and give this dude a highly reliable upper again… with a 10.3” barrel setup that’s already been proven to work in every corner of the world.

[Sparky5019]

Well, I literally just spent the last two weekends teaching our Armorers Course to students, and wrote most of the slides we use on dwell time and port sizing…

I don’t know why you seem to think that telling someone that they’ve made a mistake that can not be reliably corrected is telling them to piss off. Part of being trusted in your field is knowing when to tell people no to a bad idea.

It’s highly unlikely that you’ll find a way to make this reduced length barrel reliable now, and simply replacing the barrel and gas block with known quantity items is likely the most cost effective, time effective, and reliable resolution to your “friends” woes. If 5.56mm NATO carbine gas length barrels were viable below 10.3”, I think you’d be seeing at least a couple of them floating around. That you don’t is a clue. You’ll be in experimental territory trying to force this thing to run, with what will likely be an extremely narrow window of circumstances under which it’ll actually work.

Alternatively you could fix this problem tomorrow and give this dude a highly reliable upper again… with a 10.3” barrel setup that’s already been proven to work in every corner of the world.

Dude. He was told he made a mistake. He gets it. I stated that. Literally all you offered was telling him he was wrong and to replace the system.

How about share some knowledge from all those slides about porting and dwell times so the community can take something away from this.

[Disciple]

I don’t know why you seem to think that telling someone that they’ve made a mistake that can not be reliably corrected is telling them to piss off. Part of being trusted in your field is knowing when to tell people no to a bad idea.

+1 to this!

[Sparky5019]

+1 to this!

That’s not strictly what I said. What I said was he’s been told that. He gets it. We’re past that. I refused to tell him too bad I can’t help you. I was looking for some objective technical information that would help us work with what we have.

Looks like I’m shit outta luck.

[Disciple]

That’s not strictly what I said. What I said was he’s been told that. He gets it. We’re past that. I refused to tell him too bad I can’t help you. I was looking for some objective technical information that would help us work with what we have.

You wrote:

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.

Jim D's response which I quoted is a logical reply to this. It is not "telling him to f*ck off" to inform him that the barrel cannot be economically resuscitated to a reliable condition. It is your choice to cast it that way.

[Sparky5019]

You wrote:



Jim D's response which I quoted is a logical reply to this. It is not "telling him to f*ck off" to inform him that the barrel cannot be economically resuscitated to a reliable condition. It is your choice to cast it that way.

Fair enough. The tone and intended meaning that I took from his post was tell him he made a mistake and to start over. There was no further assistance given or support for his opinion that one could not make a 10” barrel reliable.

I addressed the fact that he had been told and he acknowledged that he’d made a mistake. We then set to the task of attempting to properly open the gas port to make the barrel reliable within his parameters. That was my question to everyone here.

I’m sorry but I do not accept an answer of “just buy a new barrel” without some objective supporting data as an acceptable answer.

This is a technical sub forum. I’m looking for technical data.

I’m sorry I don’t fall into the “just scrap it and buy a new one” crowd.

I was searching for experience that people had experience with different sizes of ports and maybe some dwell time thoughts.

If it matters to anyone at this point…

The port is at 0.078” currently and the barrel is gaining unsuppressed reliability as it is locking back on the S42H2 with several different types of ammo.

[Todd.K]

Not everything in life can be un-F’d, no matter how much you want that to be. Also see sunk cost fallacy.

Tell him it’s X dollars to replace the barrel, or X dollars per hour for you to mess with it.

I would stop chasing the gas port and figure out what else might be causing the problem before drilling any more. Check the gas tube, try a regular gas block, swap in a known good BCG, check for hammer drag…

I used to make AR barrels for a living and never had any occasion to try to make a 10” run. I doubt there is SME who can give you the right gas port size.

[Sparky5019]

Not everything in life can be un-F’d, no matter how much you want that to be. Also see sunk cost fallacy.

Tell him it’s X dollars to replace the barrel, or X dollars per hour for you to mess with it.

I would stop chasing the gas port and figure out what else might be causing the problem before drilling any more. Check the gas tube, try a regular gas block, swap in a known good BCG, check for hammer drag…

I used to make AR barrels for a living and never had any occasion to try to make a 10” run. I doubt there is SME who can give you the right gas port size.

Thanks for the info.

Everything otherwise is in good working order. We’ve had the cost discussion; I’m not concerned about that at this point.


I was thinking that 1/4” of length wouldn’t be that difficult to compensate for in port size.


Does anyone have data on what pressure loss that 1/4” might account for?


IIRC the port pressure for a carbine length has system was something like 15k psi. I can’t recall but I read it on here years ago.

[Clint]

One item that is significantly reduced with very short power time (dwell time) configurations is the ability to function well suppressed and un-suppressed without any other changes.

Drilling out the port without any idea of what size it should be for a given configuration is not recommended.

It is now larger than anyone would have recommended, so don't do any more of that.

The upper may be able to run suppressed and un-suppressed by swapping out the extra strong S42 and or H2 buffer for something normal.

The AGB should be dialed down when the suppressor is attached.