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Under-gassed AR10

slothlacrosse

slothlacrosse

Sergeant
Full Member
Minuteman
Oct 19, 2010
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Cut my AR10T's barrel from 24 in to 18 in.

it ejects the spent case but doesn't feed the next round from the magazine.

so what should i open the gas port up to?

or is there something else that i could/should do first?
 
Re: Under-gassed AR10

The Fed 168 should have no problem running the AR10 even with a 18"

When the barrel was cut was the gas block removed? If so have you checked the alignment. Just saying before you go reaming the gas block or barrel port check.

Again another option is go to an adjustable gas block like the JP
 
Re: Under-gassed AR10

First things first before you go opening up the gas port which is something that isn't always an easy fix you go too far...

Did it run properly in the same setup, ammo, etc. (but for the 6" shorter barrel) before the cut down?

I know you said you are running 168gr FGMM, but does it do this with all ammo or just that one load?

How is the rifle currently setup (buffer/spring/gas block/etc.)? I ask this because if you are running any "non-standard" component like a heavy buffer, XP spring, etc., that could be an issue which is a quick fix.

Have you checked the gas block alignment to ensure that it is properly situated over the gas port and not a little off which would prevent proper operation?
 
Re: Under-gassed AR10

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: M25BeastShooter</div><div class="ubbcode-body">What gas block are you running?</div></div>

PRI low profile gas block
 
Re: Under-gassed AR10

Very often short stroking in an AR-10 type platform is actually over gassing. This happens because the overgassing is actually unlocking the bolt too soon and trying to extract the case before chamber pressures are fully dissipated. The case is still compressed into the chamber walls and create a great amount of friction which slows the extraction process greatly. This issue with extraction timing is made worse by shortened barrels on the .308 AR's.

When this happens you'll sometimes see tweaked rims on the cases.

Anyway if you haven't already opened up the gas port then you shouldn't open it up just yet. When/if you do, an adjustable gasblock would be the best bet for proper tuning.
 
Re: Under-gassed AR10

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ORD</div><div class="ubbcode-body">First things first before you go opening up the gas port which is something that isn't always an easy fix you go too far...

Did it run properly in the same setup, ammo, etc. (but for the 6" shorter barrel) before the cut down?

<span style="font-weight: bold">ran just fine with 168 gr FGMM before the barrel was cut down</span>

I know you said you are running 168gr FGMM, but does it do this with all ammo or just that one load?

<span style="font-weight: bold">i don't know. i currently have a shitload of 168 gr FGMM for my bolt gun and didn't think to pick up anything else </span>

How is the rifle currently setup (buffer/spring/gas block/etc.)? I ask this because if you are running any "non-standard" component like a heavy buffer, XP spring, etc., that could be an issue which is a quick fix.

<span style="font-weight: bold">i used all armalite AR10 parts on the lower</span>

Have you checked the gas block alignment to ensure that it is properly situated over the gas port and not a little off which would prevent proper operation?

<span style="font-weight: bold"> on my way down to the bench to double check</span>
</div></div>
 
Re: Under-gassed AR10

I had a similar occurrence to this at the range yesterday. Shooting the new GAP-10, I have had no function issues to date other than a mag feed issue with DPMS magazines (use P-mags). I was running some reloads with 168 smk's over IMR 4895 and on two out of ten rounds the extractor literally ripped a section of the case rim off, leaving the case jammed in the chamber (pain in the ass). Despite the great accuracy with this load I will not be running it through the gun anymore.

I assume this is an over-pressure issue due to not enough gas escaping before the bolt carrier/ extractor pulled back on the spent case. Correct?

BTW. This is an 18.5"-barrelled big boy AR.
 
Re: Under-gassed AR10

everything's lined up.

i just checked the cases and there are no signs of over pressure (no flattening, sheering, pulling, etc).
 
Re: Under-gassed AR10

That's good. Sounds like you haven't opened it up yet. I think it probably would need to be opened up a tad. It might be worth asking Randal of AR-15 barrels what port size would be best. He seems to have dealt with this sort of thing in the past.

When it's opened up, it might still run fast and have off timing. (In order to fully cycle.) In that case, it's time to play with buffer weight. AR-10's tend to be way more finicky than the AR-15 when it comes to port size and location. This is why it would be preferable to just get a new barrel at a desired length then chopping one down.
 
Re: Under-gassed AR10

<span style="color: #FF0000">POLISH</span> the chamber.

Tee shirt fabric taped/wrapped around a sturdy rod sections chucked into a drill inserted not too deep into the chamber...

keep it blunt to avoid the throat.....

apply flitz or chrome polish to the cloth with motor oil or your handy lubricant...

run it to a count of 10....

wash it out and keep the chamber clean ABSOLUTELY

no matter what else you abuse about the gun

<span style="font-size: 17pt"><span style="font-family: 'Times New Roman'">"MAINTAI</span>N AND CLEAN THE CHAMBER"</span> -The AR gods

P.S. ...and tumble loaded ammo or remove case lube somehow. That crap will stick a case in the chamber.

 
Re: Under-gassed AR10

It sounds like dwell time may be your issue, but polishing the chamber is not a bad idea prior to opening the gas port diameter. You previously had more barrel pas the port equating to more time for the pressure to stay elevated and impart inertia to the carrier. With less barrel past the port, the pressure does nto stay elevated as long - dwell time. Before opening the port, try the polishing routine first then open the port by .005"- .010" max over the factory value. That is how I got my custom rig to function.
 
Re: Under-gassed AR10


"...is not a bad idea..."

is simply a negative way of expressing:

"...is a good idea...."

Of course polishing the chamber on a semi is a good idea. The better AR smiths do it for certain because doing so works.
Polishing the bolt face is too by the way as well as the extractor face.

Also, I would tend to be sure the rounds are poised correctly in the magazines before tampering with the gas port.

Moreover, I would snip a coil or two off the buffer spring too but one at a time. If you go too far, replace it with a Tubb CS spring anyway.

Then, a JP adjustable isn't much cheese.
 
Re: Under-gassed AR10

I'm going to assume this is the upper you bought from me. What's the round count? It only had 40 through it when you got it. Are you running it WET? Is the PRI adjustable? AR-10 gas ports usually run .082 - .085, a number 45 bit would fit in the port if correct.
 
Re: Under-gassed AR10

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: fngmike</div><div class="ubbcode-body">I'm going to assume this is the upper you bought from me.

<span style="font-weight: bold"> it is the upper i bought from you</span>

What's the round count? It only had 40 through it when you got it.

<span style="font-weight: bold"> 60 rounds through it since i've had it. 25 through it with the 24 inch barrel (no problems) and 15 through it since i had the barrel cut</span>

Are you running it WET?

<span style="font-weight: bold"> yes, like a sow in heat</span>

Is the PRI adjustable?

<span style="font-weight: bold">non-adjustable </span>

AR-10 gas ports usually run .082 - .085, a number 45 bit would fit in the port if correct. </div></div>

i'll polish the chamber tonight or tomorrow and see what happens.
 
Re: Under-gassed AR10

This upper was run on my only complete lower and yes, it had to be dripping wet. So did it's twin carbine upper, but I'm guessing that with a mid-length port they have more gas to begin with. I know I choked out my first 15 build in less than 60 rounds due to lack of lube.

I've never seen anybody say hey, my carbine is undergassed, and don't believe I've ever seen it stated what size that gas port is. But it almost makes me wonder if sometimes somebody in a factory forgets to change the bits when they're drilling the ports on different length barrels.

A lot of the arfcommer's go crazy when the thing doesn't cycle perfectly oob, and start drilling barrels like they're partnered up with OPEC. You said 60 rounds, but you say 25 and 15, which is 40. Either way 80 or 100 now, this might be one which has to go double that to break in properly. Polishing the chamber certainly can't hurt.

I was reading up that some benchresters use graphite impregnated oil called Lock Ease to break in their barrels. And by break in, I'm pretty sure they're just swabbing the barrel and go and shoot. I'm going to try that on the carbine and see if it's any slicker. Regardless if this thing turns out to be a POS, I'll make it right for you.
 
Re: Under-gassed AR10


Remove the buffer spring and apply a lot of grease to it. Replace it.

Coat the carrier with grease.

Coat the bolt with grease.

Apply grease to the lugs.

Grease stays put far better than oil.

Oil is for combustion engines' crankcase.

While you're greasing your AR plop about two tablespoons down around the trigger but not up on the hammer because you don't want to add weight to the hammer.

ARs want lubrication. All metal parts want lubrication. Lubricate them.

Spare the lube, spoil the gun.

Using 400 grit sandpaper, remove the phosphate from the carrier's four rails that contact the receiver as it cycles. There are two on top and two on bottom. Then grease the rails.

Your carbine just needs breaking in.
 
Re: Under-gassed AR10

I drilled my 20" build to .0935" and it runs perfect with an Armilite buffer spring, tube and rifle length buffer. It came with a hole of around .080" when I got the barrel in the white. You have to use pin gauges to see where it is now and sneak up on the correct size. Over gassing it will cause more problems than under gassing and often leads to people drilling even bigger.

See picture.

[img:center]http://im1.shutterfly.com/media/47a1da02b3127ccefcbb0f0521ce00000030O00AZOGrVwzZOGIPbz4K/cC/f%3D0/ps%3D50/r%3D0/rx%3D720/ry%3D480/[/img]
 
Re: Under-gassed AR10

To go from a 24" barrel and cut it back to 18", I would assume it functioned with the 24" perfectly and it's a rifle length gas tube? You would definitely be under-gassed. You could try swapping out the stock Armalite buffer spring with an AR15 buffer spring. That just might get you there and if it hangs up on the loading cycle, then I would swap the springs back to original and open up the gas port (a little at a time, .002-.003" can be all it takes).

I've contoured, threaded and chambered over 50 AR15 and AR10 barrels for builds in the last couple years and drilled/reamed every port then test fired for function. My bet is you're under size on that port. Every 24" barrel varminter I've built didn't require as much port as the shorter barrels (on the same rifle length gas system). Now the carbine and mid-length systems are a whole different animal and tend to be smaller yet.

I'm in the process of building a 6X47 Lapua AR10 right now with a 26" barrel and I went with a 2" longer than standard length gas system for that one.
 
Re: Under-gassed AR10

i took it out today.

i greased the lugs, bolt, bolt carrier, and trigger group (but not the hammer).

fired 100 rounds.

the first 2 were fine.

3-100 failed to feed

rounds 7, 48, 52, and 91 failed to eject

i also noticed that 14 spent cases plopped right next to my rifle. the other 82 collected in a pile about 6 ft away.

still no flattening, sheering, or pulling

i'm going to polish the chamber again and see what happens.

FWIW- this gun is an absolute hammer and the AAC brake i have on it is obnoxious (for me and the people around me) but effective
 
Re: Under-gassed AR10

Sloth, take a picture of your gun for us. We're guessing. We don't know if its a rifle or carbine, whether the gun has a rifle buffer spring in a carbine buffer tube,.....etc.

I'm willing to bet two coils off the buffer spring will fix the gun.

Are these malfunctions occurring with one mag?
Do they occur with other mags?

But I'd like to know more about how the gun, this time, failed to eject. That issue wasn't mentioned in the beginning. Trying to help. When it jams take a picture.

wink.gif
 
Re: Under-gassed AR10

100_1018.jpg

(i keep rotating it in photobucket and saving it and it won't change. i hate photobucket as much as sixpack 340 hates rugers. AHHHH!!!)

rifle gas tube/buffer/spring

the failure to ejects were something that didn't happen the first time out with the 18" barrel.

there weren't any marks on the cases that didn't eject. it looks like the spent case simply fell back into the action and the bolt tried closing on the spent case.

it wasn't jammed in. i removed the magazine and pulled the charging handle back and the spent case fell out.

it's been doing this with different magazines (all armalite).
 
Re: Under-gassed AR10


Cool. You're getting there. Fine carbine...just needs a little tuning...like a few coils off the buffer spring....one at a time, from the rear of the spring.

Try this; one round at a time in the mag. Does the bolt remain locked open after firing that one round? If not, because the gun functioned before clipping the barrel, clipping the spring will likely be the remedy. I replace my buffer springs with Tubb CS springs.

Or, if the bolt locks back but fails to strip off a fresh round from the magazine the magazine lips might need opening to allow the round to poise higher to be contacted by the bolt and stripped off properly.

Testing this could be done in the house by <span style="color: #FF0000">removing</span> your firing pin and checking feeding with rounds in the mag?? Probably make others around nervous. Still doesn't sound right, does it? Yeah. Better not.

Also, verify the gas block is secure. I glue mine with <span style="color: #CC0000">red thread locker</span>. If its loose and moving around you will get variations in ejection like you're experiencing.
Verify the ejector and extractor function properly and aren't fouling. I experienced an ejection issue once with an AR10. My ejector was fouling on a burr.
 
Re: Under-gassed AR10

Try a plain GI M16 spring. M16 springs are less stiff/heavy than AR-10 springs and should be just what you need.

Don't remove steel (i.e., enlarge the gas port) unless you absolutely have to.
 
Re: Under-gassed AR10

before i went out yesterday, i marked the gas port location with a white sharpie and a straight edge.

i also put a line down the front of the gas block.

everything lined up when i started shooting, and everything was in the same place when i finished.

i'll go downstairs to polish and grease it up again. we'll see what happens.

i'd like to get another 50 rounds down the tube before i snip the spring.
 
Re: Under-gassed AR10

I believe there is a 200 or 250 round break in period on the Armalites. Recommendations are to keep it slopy wet and clean very well. Some guys are using Mobile 1 as it sticks very well for an extended period. I've heard after 250 it runs great and hammers.
 
Re: Under-gassed AR10

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Hawk45</div><div class="ubbcode-body">I believe there is a 200 or 250 round break in period on the Armalites. Recommendations are to keep it slopy wet and clean very well. Some guys are using Mobile 1 as it sticks very well for an extended period. I've heard after 250 it runs great and hammers.</div></div>

we're @ 200 rounds down the tube now. hopefully another 100 rounds will get things going... and then it's time to start tinkering!
 
Re: Under-gassed AR10

dropped an AR15 rifle length buffer spring and it's running like a champ.

leupold mark 4 arrived the other day too.

time to go hog hunting.

thanks for the help.
 
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