Need advise on tuning an AR with jp low mass.

[Thehotrodpig]

So I put together a rifle and am having issues tuning it.

18" criterion rifle length gas barrel

Adjustable gas block

New Jp low mass bcg, jp enhanced bolt

A2 stock

Rifle buffer.

Stock a2 spring

So I started out with a stock rifle buffer. No matter how much gas I ran it would still short stroke and failed to pick up next round.

Next I slowly removed weights from the buffer and reliability increased the less weight I had in it. Eventually I removed all weight and it runs great but does not lock open after the last round.

Just to see what happened I grabbed a buddy's full weight carbine buffer and threw it in. It is heavier than my empty rifle buffer by a bunch (technical term). The gun ran flawlessly and locked open on an empty mag but makes a weird wobbly spring vibration when it cycles.

Rounds are Remington umc green and white box 55g stuff. I get it free from work so I really want to stick to this ammo at this point of my 3 gun endeavor.

From what I can tell there are no gas leaks and the block is property lined up and spaced off of the barrel shoulder.

I am a little confused as to what I have going on here. All I can guess is that the rifle buffer is bouncing back faster than the mag follower can move up and the shorter carbine buffer is going further back allowing the follower to get there. Am I close? I am still at a loss as to why the rifle buffer never reached a point of reliability as I progressed through the weight removal.

I foresee the "just get a jp low mass buffer" response which I am not against doing but I really want to understand what is going on before just buying more parts.

Thank you in advance for any help you may offer.

[thegunnerd]

I wish i could say , jus get the low mass buffer , but i had the same problem and I HAVE a jp low mass rifle buffer. Just doesn't want to lock back. Throw in my full mass carrier, perfect.

We have very similar systems as well. A2 , 18" barrel etc. I even got the jp flat wound buffer spring , i swapped out gas blocks to a seekins adjustable , swapped many dif springs , tuned the gas block all the way up.

I started getting slam fires so i put a titanium firing pin in it, no more slam fires, but no bolt lock either. I've been haggling with this low mass system for far too long and have yet to trust it enough to use at a match.

[kurtm]

"I am a little confused as to what I have going on here. All I can guess is that the rifle buffer is bouncing back faster than the mag follower can move up and the shorter carbine buffer is going further back allowing the follower to get there. Am I close?"

More than you think and more than likely you are doing a good bit of damage to your lower receiver doing this!!!

The rifle length buffer stops against the back of the rifle length buffer tube. When you just drop in the shorter carbine buffer the only thing that stops rearward movement is the gas key on the carrier stricking the buffer tube AND rear receiver ring where the buffer tube screws in. I saw a guy split his lower receiver ring in 80 rounds....DON'T DO THIS!!!!!!!!

I would check the buffer tube for an obstruction at the back! I have seen this type of malfunction when guys put on an A1 stock, but used the A2 butt stock screw which is 5/8" too long and would stop the buffer from going all the way back! I have seen guys use the wrong screw on A.C.E. stocks as well with the same effect.

[Thehotrodpig]

Ya I realize now the short buffer was a no no. Did some more reading last night when I got home. I didn't put many rounds through it that way and luckily it doesn't look like I damaged anything. I will check on the screw length and obstructions. If I had a screw that was too long would there be some sort of witness marks on the buffer?

[StealthyBlagga]

I know you want to keep shooting the free ammo, but in the past UMC ammo has had a reputation for being on the weak side. I'd suggest you try a couple of boxes of M193 just to be sure it's not the ammo.

[Alaskapopo]

DON'T PUT A CARBINE BUFFER IN A RIFLE EXTENSION TUBE!!!! You will damage your lower receiver.
Pat

[Thehotrodpig]

DON'T PUT A CARBINE BUFFER IN A RIFLE EXTENSION TUBE!!!! You will damage your lower receiver.

Pat

Thank you but we covered that a couple posts ago.

[shooterready]

You are possibly over gassed. Turn the adj about two turns out from full closed and then work your way back out until you bolt stop on an empty mag. If it ran with a heavier BCG than that is most likely the issue. An over gassed gun will act like one that is under gassed.

[Thehotrodpig]

I know you want to keep shooting the free ammo, but in the past UMC ammo has had a reputation for being on the weak side. I'd suggest you try a couple of boxes of M193 just to be sure it's not the ammo.

Shouldn't I be able to get the gun to run with this ammo I have? All my other ar's along with our entire departments ar's run with this ammo. An AR with a lighter bcg tuned properly (which it obviously is not) should have no problem running with ammo that standard ar's eat up right?

[Thehotrodpig]

You are possibly over gassed. Turn the adj about two turns out from full closed and then work your way back out until you bolt stop on an empty mag. If it ran with a heavier BCG than that is most likely the issue. An over gassed gun will act like one that is under gassed.

This is a new build. Never tried a standard bcg in it yet. It ran with a heavier carbine buffer (my rifle buffer was empty). Yes I know that it was not wise to try a carbine buffer in a rifle tube. I've been appropriately scolded and will never do it again.

I will check the buffer tube tonight for obstructions. That seems like the best advise yet.

[bladewurk]

Remington Ammo is known to be weak and will short stroke in most but the advice above is right on of course

[openclassterror]

As I recall, Criterion barrels tend to run toward the small end on gas port sizes in the barrel itself. Hate to overstate the obvious, but if the hole in the barrel is too small, you could run an adjustable gas block with a quarter inch hole and still not get enough gas. Many off the shelf ARs are over-gassed, just so they will function with crummy ammo. A quality barrel typically comes with a correctly sized port to prevent battering with full-pressure loads in a fixed gas block. What is the port size on your barrel? We did a bunch of prototyping on an adjustable piston system with settings for suppressed, normal, dirty, and "wolf". What we found was that a standard carbine length gas system needed the port in the barrel increased to about .075 (.062 is about typical) to cycle reliably with low power ammo. We increased the piston port to almost 1/32 oversized before we realized that the barrel itself was the throttle.

Typical rifle-length port size on a 20inch barrel runs .096 to .105, though I have seen ports as big as .110 from some manufacturers. 18 inch barrel should be similar, but nearer the big end. Keep in mind that an 18 inch barrel has 2inches less DURATION of gas pressure, therefore needs a little more gas than a 20. Your department likely is running carbines, which tap gas at higher pressure (closer to chamber), or recycled military rifles (20 inch barrel) which are more forgiving. UMC is known to be lower pressure than mil 5.56.

Tom

[Thehotrodpig]

Thank you Tom, that is great info. I will check the port size. You are correct, our department runs colt carbines.

I already checked the buffer tube for obstructions and there were none.

I still don't understand why it ran fine with a standard carbine buffer and not with an empty rifle buffer. The carbine buffer was heavier and yes I know I shouldn't have done that.

[TRUBL]

OK.....what Kurt said....want to see if the bolt goes all the way back?

Take the upper off the lower.

Remove the bolt carrier from the upper

Insert the bolt carrier into the buffer tube (yes....you are going to push down on the buffer and spring until it stops)

Does the carrier (not the bolt) go back and behind the bolt catch? If not.....you have some like maybe a screw that is too long.

Does the key on the carrier hit the back of the lower? If so.....you got the wrong buffer in.

Tim

Edited December 14, 2013 by TRUBL

[openclassterror]

The reason it ran with the carbine buffer and not the rifle buffer is that the shoulder that the spring rests against on the rifle buffer is partway down the body, and the carbine buffer spring rests against the underside of the buffer head. Therefore with the rifle buffer installed, the spring at rest is already compressed about an inch and a half further, and therefore more highly tensioned to start with.

Tom

[Thehotrodpig]

OK.....what Kurt said....want to see if the bolt goes all the way back?

Take the upper off the lower.

Remove the bolt carrier from the upper

Insert the bolt carrier into the buffer tube (yes....you are going to push down on the buffer and spring until it stops)

Does the carrier (not the bolt) go back and behind the bolt catch? If not.....you have some like maybe a screw that is too long.

Does the key on the carrier hit the back of the lower? If so.....you got the wrong buffer in.

Tim

I did that last night. No obstructions and the screw is not protruding into the tube.

[Thehotrodpig]

The reason it ran with the carbine buffer and not the rifle buffer is that the shoulder that the spring rests against on the rifle buffer is partway down the body, and the carbine buffer spring rests against the underside of the buffer head. Therefore with the rifle buffer installed, the spring at rest is already compressed about an inch and a half further, and therefore more highly tensioned to start with.

Tom

That makes sense. I will measure the gas port size tonight if I have a chance. Would it be advised to play with springs at this point?

[openclassterror]

It is always a possibility to mess with the springs. My biggest worry about running non-standard components like that is that if the spring breaks in an out-of-state match and you need a funky spring, chances are that nobody has one you can swap in. If you have to go that route, I would advise making a duplicate spring for your range bag, and test it for function before you put it in as a spare. If you have to open up the barrel port, there really isn't a down side since you already have an adjustable gas block to throttle it back down. Just make sure that whoever does the work deburrs the bore. If you have to exceed .109 diameter, or if the hole is already over .105, I would look for other causes before drilling out the barrel. Keep in mind that a fixed gas block has a maximum hole diameter of .159, and most rifle length gas tubes have a .125 hole to admit the gas into the tube. So, going over .109 or so just moves the smallest restriction to another spot. I don't know what the smallest restriction is in whatever adjustable gas block you have, but some have mentioned before that occasionally you find one with a big burr or a hole not drilled completely thru the passage, and the gas block itself has a restriction. Worth a complete disassembly and cleaning to check if you run out of other ideas.

Your parts list looks good, I think its just a little tweak that's missing, not a major parts swap. I run a JP low mass bolt and buffer, standard spring, and fixed gas block, but with a hybrid gas system on a 16 inch Noveske barrel with mid gas port. I have had no cycling issues whatsoever, and only clean it maybe every thousand rounds or so.

[openclassterror]

One other thing touched on by another post, but not pursued, is the possibility of over-gassing. I don't think that is the case, as it should not have improved by removing buffer weights, but if the bolt is reciprocating too fast, the follower can't lift the cartridges fast enough to get in front of the bolt and pick them up. The other reason it doesn't seem to fit is that the bolt cycled better as it lost weight but still wouldn't lock back. The easiest (read laziest) way to diagnose the is to have someone stand to your right and watch the ejection cycle as you shoot. Have them wear safety glasses. Also, where is your brass going when it ejects?

Tom

[Pat Miles]

Just a boat load of GOOD info here! I'm trying to take all of this in so if and when I have problems I'll have a clue as to where to start troubleshooting.

Pat

[Rob Tompkins]

I have a 20" barrel with rifle length gas system that combined with standard rifle buffer, spring, buffer tube, commercial BCG, etc. would run full power ammo just fine but would not run soft hand loads or lower power bulk 223. Barrel port is 0.092". Even with a JP low mass BCG and buffer and adj gas block wide open it wouldn't run lower power ammo reliably.

If I put that exact same upper w/ BCG on a lower with CAR buffer & buffer tube it would run ANYTHING and EVERYTHING. If I put a CAR upper on the rifle lower it too would run ANYTHING and EVERYTHING!

[Thehotrodpig]

I have a 20" barrel with rifle length gas system that combined with standard rifle buffer, spring, buffer tube, commercial BCG, etc. would run full power ammo just fine but would not run soft hand loads or lower power bulk 223. Barrel port is 0.092". Even with a JP low mass BCG and buffer and adj gas block wide open it wouldn't run lower power ammo reliably.

If I put that exact same upper w/ BCG on a lower with CAR buffer & buffer tube it would run ANYTHING and EVERYTHING. If I put a CAR upper on the rifle lower it too would run ANYTHING and EVERYTHING!

I might try putting my car lower on this and see what happens. I prefer a fixed stock on my 3 gun rifle but the car lower might have to work.

I am going to start first with removing the gas block, measuring the port and assuring that the tube and gas block are clear of obstructions. I do remember noticing that the hole in the barrel was very small compared to the large hole in the gas block.

Thank you everybody for all the help and advise. I will get back with my findings.

[Thehotrodpig]

I don't really know the history of the spring since it has been sitting around with my a2 stock for the last decade. If I wanted to put a new spring in there to reset my baseline, what should I try? Standard a2 spring or one of the fancy aftermarket types? Is there a recommendation for my app? Not a custom shorter one at this time.

[Thehotrodpig]

Ok, so I took the gas block off and the block and tube were free of obstructions. The hole in the barrel appears to be right around .091. It was difficult to get a perfect measurment with my calipers but I am pretty close. Does this sound like it is on the small size? The barrel is a criterion 18" hbar with rifle gas. .750 under the block.

Edited December 14, 2013 by Thehotrodpig

[openclassterror]

I have only one 18"/ rifle gas barrel in the shop at the moment, and it is a DPMS S.S. HBAR contour barrel (This is a misnomer, as Colt never made an HBAR with this barrel length and gas system. But I digress.....). Port size is .098. Keep in mind that the back (inside) jaws on most dial calipers have a small flat on the measuring face, and in a small round hole they usually read small by about .002 or so. Gage pins are the best way to measure, but most guys don't keep a set in their tool box. You can also use drill bit shanks as gages, although there are gaps in between sizes. Measure the actual drill shank, as they are typically a thou or two under flute size. The most common port size I have seen on rifle length gas systems is .096. That seems to be standard for DPMS 20s, and Kreiger uses .096 on their 20inch barrels. Derrick Martin of Accuracy Speaks was putting .110 ports in his service rifle barrels last I saw, and that is at the very big end of where they should be. Keep in mind service rifles are typically built as heavy as they can be, and usually use max heavy (M-16) carriers. These are most easily identified by the lower rear part of the bolt, behind the hammer slot. On a commercial AR carrier, there is about 1/2 inch of steel across the bottom of the carrier at the rear, on an M-16 carrier it is more than an inch long. When these carriers are used commercially they are typically called "Match" carriers to separate them from the full auto parts stigma. In and of themselves, they do not contribute to full auto usage other than the extended tail area is needed to trip the auto sear. However, many people think the added mass contributes to reliability in forcing the bolt closed on balky ammo. So you see them on a lot of semi-auto only rifles as well. This background info doesn't apply to your situation, as you are going the opposite direction (light carrier), but might be helpful to somebody. So in answer to your question, your port seems a little bit on the small side, especially since you have less barrel length under pressure beyond the port. I would have no qualms upping to .098 or so, especially since you already have an adjustable gas block.

Tom