AR-15 In 7.62x39R, Problems...

J

JeepHammer

Moderator
I have a Colt 7.62x39mm Upper, 20" barrel.
This thing has NEVER cycled the bolt fully.

I've opened up the gas port in the barrel,
The gas port in the front sight,
I've used the largest diameter gas tube I can find,

I've went the other way, restricting the gas to the tube.

I've tried different buffers, springs, ect.

Nothing works, the bolt simply isn't getting fully to the rear to strip the next round off the magazine.
(Yes, Using 7.62x39R Mags from Colt)

Since I bought the barrel/bolt/gas block/sight, all from Colt, I would *Assume* they would work together in standard AR Upper receiver.
It's a Colt factory flat top upper receiver.
I've tried other uppers receivers, and none worked no matter how much tweaking I did...

I've seen a bunch of 7.62x39R uppers that did function,
But all of them were 16" barrels, gas block moved backwards on the barrel.

Anyone have this same issue and how did you fix it?
Am I going to have to cut a gas block pad and drill a gas hole at the short barrel length to get this thing running?
Or is there something I've been missing all this time?

I've never had an AR I couldn't get running before,
This thing defies all attempts to get that bolt carrier/bolt to the rear...
 
I had the same issue with a Model 1 Sales upper a few years ago. It never cycled from day one. I contacted them and they had me send the bolt carrier group back to them. When it was returned, I reassembled the rifle and it cycled fine and still does.
I asked them what the issue was and was told it was "carboned-up". I never understood that since only 10 rounds were run through it. And I cleaned it very intensely after every test fire session.

Good luck.
 
Sounds like BS to me...
I think they might have screwed up and just changed out your bolt/carrier to get you running.

------

There isn't supposed to be any difference between 7.62 and 5.56 bolt carriers,
Colt lists them as the same part numbers,
And I've tried different gas rings, different bolt carriers, ect.

Checked head spacing several times, and it's correct.
Checked the bore to make sure it would shoot .310 ammo, it's correct...

Mag changes, upper changes, bolt carrier changes, gas ring changes, gas tube changes,
Buffer/Spring changes,
This has got me baffled!
 
Have you tried running the whole upper on a different lower? It seems like you have tried everything else... maybe there is a spec issue between the two causing it to bind somewhere.
 
I've had it on three lowers, changed upper receivers twice,
(Once was from carry handle to flat top),
There is just something I'm missing here,

I've tried several different brands of ammo,
Cleaned the crap out of the bolt carrier bore,
Even started with a tight bolt carrier and honed it in increments.

This is just driving me nuts...
 
Could it be problem with the alignment between the gas port in the barrel and the orifice in the front sight?

BTW, I believe the cartridge is 7.62x39, or the AK round. When I saw 7.62x39R I thought it was a new rimmed cartridge.

Good luck.

-TL
 
JeepHammer,

How far back from the muzzle is the front sight? Have you looked to see if the bolt is rubbing on the upper, or anything else, that could retard it? What about the gas rings, and the sealing between the bolt key, and the bolt?

The gas pressure will drop the farther down the barrel it goes, but at 20 inches, it should still be plenty enough to operate the bolt. The thing that could cause it, would be the gas port too close to the muzzle, however, the factory position should work. Is it possible for any gas leaks to be around the front sight base?
 
Also, are you using an AR handguard end-cap or a free float tube? I ask because (and you may have already checked it) if your bbl gas hole is set forward to allow the use of a handguard retaining en-cap, but your pushing your gas block against the shoulder on the bbl- your holes aren't lining up. OR- could it be the other way around? Using an endcap, when the bbl hole was drilled short NOT to allow the use of the cap?
 
BTW, I believe the cartridge is 7.62x39, or the AK round. When I saw 7.62x39R I thought it was a new rimmed cartridge.
Click to expand...

Close. I believe the cartridge is 7.62x39 Russian. Hence the confusion.

The barrel on mine is 20 inches with a low profile gas block and a free float tube.
 
10-96,
I'm using the 20" hand guards, the end cap is in place.
I measured the distance from gas block hole to step on the barrel, and it *Should* be dead center the gas port in the barrel...

I've opened up the gas port in the barrel as much as I dare to,
I've opened up the gas port in the gas block/front sight,
I've drilled the gas block/tube to make sure the holes line up correctly between gas block and gas tube,

Everything else has been changed out with the exception of barrel and chamber nut/bolt.

The bolt simply isn't reaching full rearward travel.
The head space is correct, the lugs are lapped,
The bolt/bolt carrier works freely,
The rifle will strip every single round off the mag and chamber it when I use the charging handle,

But that damn bolt carrier simply will NOT move rearward enough to pick up the next round off the mag.

It ejects fine, no issues with ejection,
The bolt just bites into the side of the next round in the mag instead of getting behind it.

I did the old rolled paper tube in the buffer spring trick,
And the buffer lacks getting to the rear of the buffer tube by about 1-1/4"

This is with different lowers, different uppers, different mags, different charging handles, different bolt carriers, different gas tubes, ect.
I've tried different gas rings, changing from factory 3 piece to a 1 piece spiral,
I've used a verity of factory ammo, not even the US made ammo will cycle.

The bolt isn't sticking in the carrier,
The rounds aren't sticking in the chamber,
The locking lugs aren't grinding anymore,

I'M STUMPED! I've never had this much trouble with an AR in 30 years of doing this...

As near as I can tell,
The gas impulse will unlock the bolt, but simply doesn't have a long enough duration to move the bolt/carrier to the rear enough to pick up the next round...

Anyone know a DEFINITIVE way to check to see if the gas port in the barrel is lining up with the gas block/sight?
 
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I have a simple test to check to see if the tube is blocked.
Make sure the rifle is unloaded and the bolt carrier group removed from the upper.
I use a piece of clear plastic tube from an aquarium air pump, (Helps if the plastic tube is a couple of feet long.) attach it to the tube inside the upper and blow into it.
If I can't hear air escaping into the barrel, I need to clean the tube or check the hole alignment.
Maybe not a proper test, but it works for me. YMMV
 
You can check the black soot mark on the inside of the gas block. The mark is made by the gas from the gas port. The mark should be centered around the orifice in the gas block.

-TL
 
Try using a Rock River 7.62X39 AK magazine --- which is the only type of magazine that functions in it's AR --- if not....the magazines that you're using, might have to be tweaked.
 
I am going to have to go on the side of mis-aligned gas port. either fault of yours, or fault of the barrel or gas block being out of spec. do you have a good set of calipers to get a good reading of the port from barrel and port from outside of gas block? are there carbon or burn marks around your gas block? have you removed the gas block to make sure it isn't obstructed somehow? I have a cheap barrel that just barely didn't line up, by thousandths, and it would run some strong ammo fine and weaker stuff not so good. I had to open up the hole a little on the gas block to get complete coverage over gas-port and now runs great. the block could just be a bad fit around the barrel, but then you should see the carbon marks coming out from under the block. i'll bet you is has something to do with gas...good luck.

just blowing air into it will let you know a little, at least if it's clogged out totally out-of alignment, but that sucker really needs to be aligned perfect to run correctly. you can still blow air through it if it's just aligned a little bit. but that could be a good place to start to see if its just clogged at least

put a pin the same size as gas port into the port, has to be close in width of the barrel, let the pin drop into the barrel. got it? okay, then put the gas block on, tip the barrel upside down and make sure the pin falls through the gas-port into the gas block. if so, you know it's aligned. make index mark on block and barrel, remove pin and re-install block by marks you made when you knew it was aligned with pin. I usually have a roll pin lying around that work for the purpose, or you can quickly make one with a drill and some sandpaper.

doesn't need to be a metal pin, could be a toothpick or whatever is close in diameter to gas port, but can move freely inside of it. if this isn't making sense, let me know and i'll attach some pics. if you do all this and your still stuck even after gas is 100% aligned, you could open gas pot slightly with dremel. you can rule out magazine by firing it with one round in chamber and se if bolt holds back, if not, it's not the mags fault.

the reason I feel so confident that this your problem is because I have built all my uppers from day one with 0 experience and little tooling. always had a problem, would just go to range and make slight adjustments to block until starting running well. I tried chalk lines and all kinds of craziness before just using a pin. if you use a toothpick or piece of wood, you can actually just set it without making index marks and break the toothpick out with the cleaning rod. but I like having an index there if I ever need to take it off again.
 
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I installed the gas tube in the gas block/front sight.
I used a drill to slightly OVERSIZE the hole in the gas block/tube to make sure there weren't any issues.

I slightly over sized the hole in the barrel, since it wasn't quite up to specifications of Colt.

I used digital caliper to measure the distance of centerline the gas block, and compaired it to the barrel/hand gurard front cap,
It should be dead center.

The only thing that *Could* be out of alignment with the gas block is side to side.
No burns on the inside of the gas block or the outside of the barrel I could see.

The gas block fits the barrel VERY tightly,
And I used sealer during a few of the installs. No leaks.

Testing the gas system might be an option,
I have a flow meter on my welder, I'm thinking of hooking up a flow meter to the gas tube and seeing if there is a restriction someplace.

Bolt carrier changes, gas ring changes, no hang up on mag or locking lugs,
Free buffer tube, no snags.

It will strip EVERY ROUND off the mag with the charging handle, but it won't pick the second round up off the mag when firing.
It cycles WONDERFULLY by hand, just won't pick up that second round off the mag...

I've even just loaded two rounds in the mag, just to see if the mag spring was being compressed too much,
And it wouldn't pick up the second round.

EVERYTHING ELSE HAS BEEN CHANGED,

Barrel, chamber nut, bolt, front sight/gas block are the only 'Common' parts that haven't been changed,

And it does the same thing with every mag, every upper, every lower, every bolt carrier,
Every spring/buffer, every gas tube, ect.
Several different brands of ammo...

I just don't think the gas pulse is long enough to throw the bolt fully to the rear...
 
I don't know then, adjustable block and widening the barrel port even more? can't say what else could help. overgas it and put a lighter buffer weight.

most people do not have these issues with x39 AR15's, so the gas is there, it's just not doing the job for whatever reason

quick test, may help, prob not. take off your brake. plug the tube and chamber and blow into the end make sure nothing is leaking. but if there isn't any carbon burns anywhere, I can't see where it would be.
is the gas tube fully locking into bolt carrier? like, long enough? the tube should be just a bit over half past the crescent in the upper receiver. maybe, just throwing ideas out here, gas block stop on barrel machined too far out so tube can't get far enough in? but i figure that would cause very noticeable blowback in your chamber
 
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There's a possibility that it's the bolt retarding the action. Check the cam slot in the bolt, to make sure the machining is smooth, and that there's no roughness for the cam pin to rub against, when it rotates to unlock. Also, look at the firing pin, and make sure that gas isn't escaping around it.

If you have sufficient gas making it to the bolt, then look at the gas tube to bolt key connection, and how they match, or whether the tube is long enough. Next, the bolt key leaking between it and the bolt, or the port in it undersize in the key. Next, the gas rings in the bolt, and last, the firing pin. Each one is a potential gas leak.

You may have to try a complete new bolt.
 
gas rings....that's a good thought. but Dixie, wouldn't all of these potential leaks makes obvious burn patterns upon close inspection? would that be a way to diagnose anyways?
 
It may and may not. The gas has had time to cool a bit, when traveling back into a rifle like this, and generally builds up over time enough to show a dark mark. Where this is a new rifle, it may not be that noticeable of a mark.

Since the bolt seems to work freely by hand, when using the charging handle, it would have to be something not driving the bolt back hard enough against the recoil spring. The bolts cylinder is there to separate the bolt, and drive it back, due to it's large internal diameter, working as a piston. Once the bolt's cylinder comes to the end of its stoke, and unlocks, the remaining pressure in the gas tube is what pushes the bolt back through the bolt key, and against the rear face of the cylinder. When the tube pulls out of the key, the pressure dissipates, but the bolt should continue back until it reaches its limit. Any leak in the bolt could keep this from happening, over low pressure.

In theory, the longest gas tube, that will just pull out of the key, by maybe 1/8 to 3/16", with the bolt completely rearward, is what one would want for correct operation, though if I remember, the gap is a good bit more than that. The tube has to come out of the key to quickly release any gas, as if it didn't, the gas would have to be pushed back through the tube, and out the port in the barrel, when it closed. That would surely cut the cyclic rate down. It will push a little air out as it is, because of the bolt cylinder closing.
 
I broke down and ordered a new bolt...

Took the key off the bolt carrier, everything lines up and is wide open,
I'm getting good gas staining/carbon on the bolt, and it ejects fine, so I don't believe it's a gas issue.

Flow tested the gas tube,
Figured the diameter/volume, set argon through the tube, got about a 7% restriction from gas tube to barrel.
That's to be expected with the 90% turn at the gas block...

This is the most aggravating issue I've ever had on an AR of any kind,
Even the AR-10, .300 WSM top ran like a champ with just a few tweaks.

You sure can't make any money on these issues when it takes MONTHS to get the thing running...
 
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