AR-15 troubleshooting...

[AJMD429]

I would post this on an 'AR-15 forum' of some sort, but

• a) I get too many 'forums' to follow-up on, so I just stick mostly with this one
  b) I don't need a lecture as to why didn't I get the $3,000 custom one some other guy has...

Anyway, I usually get more concise (and accurate) answers to even non-levergun stuff here.

I've put a few AR-15's together from parts over the years, and never any issues at all, until I did three 300 AAC Blackout uppers, and I'm still trying to shoot those three when I get the chance, to see which one, if any, I can use both suppressed with 220 grain subsonics, AND shoot 125 grains unsuppressed (and if more than one prove able to do that, which is more accurate).

However, a friend put together a one in 223 from mostly CMMG parts, a 'shorty' one 16" with 'carbine' length gas port. . . and it won't cycle. . . ! He lives in the city, so can't do much testing, and hasn't built an AR before, so he asked me to check it out.

It is tight and well made, and yet won't extract/eject the fired case.

I took it all apart, and the bolt is in the carrier with all three gas-rings installed and snug in the carrier, I can get a pipe-cleaner into the carrier's center through the riser thing that goes onto the gas tube. The bolt has an intact ejector and extractor. All moves smoothly through the upper. The spring 'looks right' and the buffer is the short 'carbine' type. I removed the barrel and reinstalled it with proper torque, and made sure the gas block has a patent passage, and that the opening was over the barrel port when it was reinstalled (a clamp-on type). When I put a WD-40 can nozzle-tube into the gas tube in the receiver, it spurts freely into the barrel when you look down it.

So. . . what am I missing. . . ?

I will try different ammo today sometime if I get a chance, but his was factory Federal FMJ, and I don't think he got hold of any 'subsonic' or anything.

[pwl44m]

I had one do that and it was indeed the buffer & spring. I was using a rifle length B&S with a fixed stock. When I got the right parts and switched them out it worked fine. It was one of My first ones. I did have it happen again but this time the gas key wasn't "staked" properly and worked loose causing it to leak pressure. That is the only 2 I have had problems with.
Perry

[Griff]

Whem you say, "...won't cycle...", what exactly do you mean? Will the bolt move far enough back to pick up the next round? If so, is it jamming that round someplace? If you load one in the mag, will it lock back on the empty mag? A kinked or bent gastube will let gas thru, but not quite enough.

I gleaned this tidbit off a similar situation on AR15.com:

A carbine spring should be between 10 1/16" - 11 1/4" , and a rifle spring should be 11 3/4" - 13 1/2".

[wecsoger]

Same deal on one of my shorties with an unknown manufacturer barrel.

Doing same thing, extractor would either rip off rim of brass or barely pull it out enough and still short cycled

Is barrel/chamber chrome lined or plain finish?

Mine was /not/ chrome lined. I very carefully polished the chamber with a piece of Scotch-Brite pad, one of the red/maroon very fine ones. Chucked a long drill bit in the cordless, wrapped the pad piece around that.

Working theory was when they 'parked the barrel there was enough build-up in the chamber to create excessive friction.

That cleaned it up enough to reliably cycle.

[Streetstar]

I had a "hobby AR" I built from a Model 1 Sales kit (hey.... don't knock 'em till you try 'em ) --- it was doing the same thing due to the gas key being loose -- I didn't re-stake it, I just dipped the screws in red Loctite and reinstalled to Guddentite torque -- not a problem since

For a defense AR or something the owner may count on someday, I would tighten them and re-stake the carrier key with a punch or something though ------

If that isn't it , i'm out of ideas

[Griff]

I would post this on an 'AR-15 forum' of some sort, but

• a) I get too many 'forums' to follow-up on, so I just stick mostly with this one
  b) I don't need a lecture as to why didn't I get the $3,000 custom one some other guy has...

What I hear you saying here is that you don't want get an AR version of Terry tellin' you, "...YOU DON'T KNOW ENOUGH TO ASK AN INTELLIGENT QUESTION...", so you're whimpin' out and askin' here?
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Hope you know I'm just yankin' yer chain!

[AJMD429]

The chamber looks ok. The bolt feels like it is moving but not enough to un-chamber the round - it pulls partly out then just re-chambers.

I'll try:

• some of MY ammo
  swapping out MY buffer/spring for his
  cursing and swearing alot

Wish me luck.

[Griff]

Will it cycle fully by hand?

[Rusty]

Check to make sure the 3 gas rings on the carrier are not lined up. Other place to check might be at the gas block. It sounds like a gas problem.

[wecsoger]

I had assumed (ahem!) that you had already checked the gas rings. Little boogers will crawl around on you when you're not looking.

On my shorty, as mentioned before sometimes the cases would even stick in the chamber, with the attendant ripping off of the rim by the extractor.

What ammo? That steel case stuff is hard to deal with all around. And I would try different ammo, that's an obvious troubleshooting step to knock down another variable.

Since you have the once-fired cases (not reloads?) I'd look them over, which I'm guessing you've already done. No scrapes, drags, gouges, grooves or distortions?

Went back and re-read your last post. Hmmmmm... If, if the extraction appears to be working, then we're back to weak gas pressure or too strong buffer/spring pressures holding the bolt closed.

Still have the problem with different ammo, I'd be tempted to try this - pop the gas tube pin, remove the gas tube. You now have a manually operated single shot. This is an opportunity to check your extraction to see if it's rough or tight.

If that's fine, then you're back to gas pressure, or lack thereof.

[wecsoger]

Adding, thank you for posting. This is one of those fun problems we can ruminate and chew our cuds on.

[rogn]

Consensus is that gasring alignment is of no consequence. Worn or loose rings however are a problem. Would it be simpler to pull the extractor and fire it, then replace and check for "stickiness"?

[Griff]

Ok Doc, it's been 2 hours... is the patient cured?

[MrMurphy]

Correct, gas ring alignment is irrelevant, the gun will run with a single one.


Gas key staking and the buffer/spring would by my first checks, next, gas port.

[AJMD429]

Well.....

• 1. My ammo. . . .
  • "Bang!". . . still wouldn't cycle.
  2. Swapped the buffer and spring with my shorty. . . .
  • "Bang!". . . still wouldn't cycle.
  3. Swapped the bolt carrier with mine. . . .
  • "Bang!". . . still wouldn't cycle.

So.....

Figgered it had to be in the stuff I didn't swap out; barrel, gas block, gas tube.

Swapped out a block and tube and. . . .

• Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . . Bang!/sproing/thump. . .

. . . gotta love those sproingy AR's. . .

[Griff]

. . . gotta love those sproingy AR's. . .

Thats why t the 1sst opportunity, I'm changing mine over to a JP "captured spring".

[JerryB]

Glad you got it SPROINGING again, Doc you sure do got a way with words.

[wecsoger]

Yes, having something by your cheek, first that's not wood and second, that goes 'sproing' when you fire just seems wrong.

So...is it the gas tube or block? The former easy to check with a piece of safety wire, the latter, I don't know. If all looks properly drilled, then a mis-alignment on the barrel?

[AJMD429]

So...is it the gas tube or block? The former easy to check with a piece of safety wire, the latter, I don't know. If all looks properly drilled, then a mis-alignment on the barrel?

I really couldn't tell.

The tube passed a pipe-cleaner clear through to be visible in the gas-block hole.
The end of the tube interfacing with the bolt carrier appeared normal.
The block was aligned well with the barrel port.
The block's gas hole and tube's gas hole were aligned.

The only thing I saw was that the hole in the gas block was large in diameter vs that of the gas-tube, so there would be a bit of extra volume there, but maybe the equivalent of another 1/4" length of gas tube at most.

Strange.

If he doesn't want the old gas block and tube, I'll probably separate them and may try them separately in another gun sometime. Or not.

[Malamute]

A buddy told me about a problem he had with his build. It was all high quality parts. He had an AR gunsmith tinker with it and it took a long time to hit on the answer.

At the factory that made the barrel (A well known quality AR maker), the carbine barrel was in the production line. It apparently was drilled for the gas port along with some other barrels, or the bit was left in from the previous batch...only carbines require a different size gas port hole. Once the difference was discovered in the port size, it was drilled to the correct size and it worked perfectly afterwards. It didn't take much size difference to fail to function from what I understand. I believe an accurate machinist drill bit size set is needed, like letter and number sizes, to determine the size of the existing hole, and to drill the correct hole.

[wecsoger]

True, that. There's a balancing act going on here, it's a shorter barrel so there's less time for the pressure to stay up, but the port is closer so you're getting higher pressure gas, blah, blah, blah. I'm not an engineer but have had a reasonable amount of luck (can't really call it skill!) in getting my builds to not only go bang but also extract and feed.

That's with AK's, AR's, FAL's and the random 1911 clone or two.

And yeah, still all ten fingers. (grin)

But I'm still curious as can be on this one. Note what AJ posted. He swapped the gas tube and block and everything started functioning as normal.

Not a heckuva lot to a gas tube. Piece of tubing, holes where appropriate. Extremely easy to see if kinked. For blockages, run a long piece of safety wire or one of those mutant pipe cleaners through. You can lay it beside another one to see if the holes are correct. Other than having an incorrect internal diameter, I can't see what could be wrong with one.

Gas block, a little bit more engineering. Not that much so. It sets on top of the hole in the barrel, obviously alignment is v. important. Otherwise in this tasking it's just there to hold the gas tube correctly so the pressure will transfer from the barrel, though the hole, into the gas block and into the hole in the gas tube so the pressure wave can head back down the barrel toward the bolt carrier.

So beats me. Good bit of troubleshooting but there's still two variables to knock down, the tube and the block. Unless both are out of spec just enough that they're working together for a failure, one is obviously bad.

[Griff]

Why adjustable gas blocks are nice... but... as with most everything else, the more complexity you add, the for opportunity for Mr. Murphy to rear his ugly head... (no you mr murphy... the famous one)!