View Full Version : I Think It's Under-Gassed?
Good evening everybody,
I just put together my first stripped barrel rifle and it's failing all across the board.
It's short-stroking, failing to eject, failing to hold bolt, failing to return fully into battery and failing to cycle the bolt at all on weaker Russian ammo.
I miced the gas port at .062" and read that mil-spec is .063 or about.
It was freezing at the range today--30degF and I just brought the one rifle to function test. I would've tested swapping parts with my known 100% rifle if I'd brought the darn thing, too cold anyway.
None of my ammo worked: WCC M855, Fiocchi 55 gr. PSP, UMC 55 FMJ, 24 gr. of TAC under a 55 gr. FMJ, etc., etc.
I read the barrel descriptions from several suppliers like Daniel Defense, they open up their 16" carbine barrel's gas ports to .075"
Should I consider opening up the gas port to .075?
GermanSynergy
01-11-11, 18:19
What rifle/barrel were you using?
How much lube did you use on the BCG?
Belmont31R
01-11-11, 18:20
What type of gas block does it have? If it wont work with any ammo there is likely an obstruction. Also check the gas key on the BCG.
A ~.62-.63 gas port should be fine for a carbine barrel.
A list of the parts in your rifle would be helpful. 75 thou's is quite large for a carbine gas system on a 16" bbl.
You say it doesn't hold the bolt back. Do you mean after the last round or by pressing the bolt catch?
Probably a gas leakage or obstruction somewhere. Take a good hard look at the BCG and gas key. Make sure its staked properly and is tight to the carrier.
Also what spring and buffer?
What type of gas block does it have? If it wont work with any ammo there is likely an obstruction. Also check the gas key on the BCG.
A ~.062-.063 gas port should be fine for a carbine barrel.
fixed it for ya...
;)
I wouldn't touch the gas port. My 16" carbine has a 0.63" gas port and can run Federal American Eagle .223 with an H2 buffer.
Might be something sticky somewhere. Might an obstruction in the gas key. If you have a known good bolt assembly from a different gun, maybe swap that in and see if it changes anything?
You left a lot of info out, but I will make a guess that it is a 16" carbine.
The correct GP size for a 14.5" M4 is .063. So if you have a 16" barrel and it has a GP size of .062, then you are fine.
If you installed the gas block, then that is most likely your problem (not aligned properly). Or you installed the gas tube upside down in the GB (my personal favorite).
C4
Do you have the holes for the gas port in the barrel and gas port in the gas block aligned correctly? Its an easy mistake that can cause a lot of problems.
Mine was off by a little (block was a tad too far back) and it wouldn't always hold open on the last round.
Let me beat the dead horse. Please come back and provide us more information than you did. This way we can help you.
I also wouldn't do any initial testing with crappy ammo.
... I read the barrel descriptions from several suppliers like Daniel Defense, they open up their 16" carbine barrel's gas ports to .075"...
I'm curious where you saw that. It's hard to believe DD would open up the gas port that wide over the norm of .063". And if we're talking a midlength gas system you'd expect something more like .059".
I'm curious where you saw that. It's hard to believe DD would open up the gas port that wide over the norm of .063". And if we're talking a midlength gas system you'd expect something more like .059".
Keep in mind that the further the distance from the chamber to the gas port, the lower the pressures and will require a larger sized gas port to operate the rifle. I believe the port of a rifle length gas system is .092"
Interesting thread. Does anyone know definitively what the port size is on an SR15?
Thanks for all your replies and patience.
That storm had me busy at work.
I'll not be giving out manufacturers names because I'm sure it doesn't matter relating to my functioning problem(s).
I've got parts swapping to do at the range first between known reliable stuff and the stuff on my newest creation to trouble-shoot specific parts and assemblies.
I'll be trying a piece of flexible vinyl tube tightly fit over the end the gas tubes of both the known good rifle and the new problem child and giving them a good puff and comparing how air moves through them.
I was hoping someone had a quick solid number--??? of an inch diameter gas-port-- gave the best all-around functioning with most civilian ammo out of a 16" carbine.
I promise to keep you guys updated, it's just that I'm a bit busy.
Keep in mind that the further the distance from the chamber to the gas port, the lower the pressures and will require a larger sized gas port to operate the rifle. I believe the port of a rifle length gas system is .092"
My understanding is that Colt uses .088 on rifles.
Leave the gas port as is. I highly doubt that's the cause of your problems. As suggested above, check your gas key. Most importantly for now, since you mentioned thatthis is a stripped barrel build, check the alignment of your gas block.
When you have the time, give us a complete breakdown of the entire build.
Actually the manufacturer has ALOT to do with it. If you want our help provide the information. If not, then continue to roll the dice until you figure it out.
All AR's are not created equally and they vary across the board.
Thanks for all your replies and patience.
That storm had me busy at work.
I'll not be giving out manufacturers names because I'm sure it doesn't matter relating to my functioning problem(s).
I've got parts swapping to do at the range first between known reliable stuff and the stuff on my newest creation to trouble-shoot specific parts and assemblies.
I'll be trying a piece of flexible vinyl tube tightly fit over the end the gas tubes of both the known good rifle and the new problem child and giving them a good puff and comparing how air moves through them.
I was hoping someone had a quick solid number--??? of an inch diameter gas-port-- gave the best all-around functioning with most civilian ammo out of a 16" carbine.
I promise to keep you guys updated, it's just that I'm a bit busy.
Actually the manufacturer has ALOT to do with it. If you want our help provide the information. If not, then continue to roll the dice until you figure it out.
All AR's are not created equally and they vary across the board.
If I'd have known you were so uptight about it I wouldn't have posted. Cheeezz!
No one is being uptight about it. All that is being said is that not all AR part manufacturers are the same. If you tell us what you used we may be able to help you pin down a part by a problematic manufacturer.
I've done it. I cheaped out on my build at first with a blackthorne complete upper. Ended up with issues cause at least partially by substandard parts. My upper is now completely different and built with components from quality companies and it runs like a champ.
Look at it this way. Its like you are going to a car forum and saying my custom built Chevy won't start, what's wrong with it? Then not giving any more information about the car.
It has nothing to do with uptight. You came asking for help and we cannot give you accurate info if you don't supply it.
If you want help, provide the info or fix it on your own. Until then it's closed.
If I'd have known you were so uptight about it I wouldn't have posted. Cheeezz!
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