I had a well known gunsmith build a custom 7.62x39 AR barrel for me using a 12" AK47 barrel (b/c it is hammer forged AND chrome lined). Had a couple of problems of which I fixed one. The other one is an over gassed system, I think. Here is the scenario:
Insert mag, chamber a round, round fires, it usually extracts/ejects fine, action chambers a new round, fires but won’t extract AND the bolt is stuck forward. I have to take a cleaning rod and tap it loose. Sometimes this happens even after the first round. Also, sometimes the case rim would be ripped off or the case rim would be badly damaged. I have tried a LW bolt, a COLT bolt and a CMT bolt with exactly the same problem. I doused the carrier, bolt and extension with lube with exactly the same results. I put in the heaviest buffer I could find with the same results (MGI RRB and a 9mm buffer). I finally took off the FSB and used a pin gauge on the gas port. It measured .091 and I am guessing it should be somewhere around .074.
What do the experts think?
If the consensus is over gassed, then I will try an Wolf Extra Power recoil spring since I have one sitting in my parts bin.
w/o a full auto lower & a stack of progressively heavier buffers for diagnostic, overgased could be hard to tell. I find it strange that the bolt sticks. Try another?
Maybe one could plug a smaller gas port or try a LMT enhanced carrier.
7.62x39 has so many different: powder, brass, pressure variences … your prob could be ammo related
Gas port size is dependent on barrel length. If it is too large I agree with Dano to contact Randall. He can install a bushing in the gas port making it smaller.
The barrel is 12" and it is a Bulgarian AK47 barrel so it is a very skinny barrel similar to pencil style AR barrels. The chamber end of the barrel was just fat enough to thread an AR extension on and it headspaces just fine. And I meant to write the gas port is .094" not .091".
You might want to check with Ken Elmore or Tina at www.specializedarmament.com they may have some data as to what Colt used on it’s 7.62x39 barrels when they made them. Different calibers, different thicknesses and different lengths of barrels all change the equation.
If it’s determined to be too large of a gas port Randall can install a bushing to bring it down to the correct size.
Overgassing wouldn’t have anything to do with the spent case not coming out of the chamber, and bolt stuck forward.
If the spent case needs to be hammered out of the chamber with a cleaning rod, the problem is in the chamber, not the gas system.
Sounds like an ammo/chamber problem to me as well.
12" barrels don’t exhibit the over-gassed symptoms nearly as much as 14.5" to 16" 5.56 carbines will.
Due to the larger bore volume of a 30 caliber barrel, the gas ports DO run larger than you are used to on 5.56 barrels.
0.094" does not sound excessively large to me for a 12" carbine gassed 7.62x39 barrel.
I’m not an expert but the first thing I’d check would be the Ammo. What brand are you using? The Brass sounds awful soft to me considering that it sometimes loses the rim. It could be that the ammo is expanding more than it should. It could also be the chamber, it may not be the correct size, how hard is it to manually cycle the weapon? The gas port size doesn’t sound to big & it doesn’t sound like to much overgassing. But I’m no expert.
I have a COLT 16" HBAR in 7.62x39 that I cut down to 10.5", threaded and opened the gas port. I checked it last night and the gas port is .087" whereas the 12" is .094". The 10.5" runs great on semi and full auto. So the 12" pencil barrel is .006" larger than 10.5" hbar barrel. I read on the Maryland site that hbar gas port would be bigger than a pencil gas port due to thickness of barrel.
I also dropped a round into both the 12" upper and the 10.5" upper to see if the round stuck out further in one barrel or the other. In the 10.5" bbl, the round stuck out of the chamber .021" and in the 12" bbl, the round stuck out of the chamber .018". I did this to see if I could have a headspace issue.
Does any of this information change your mind as to a chamber issue?
Randall - On the 12" barrel, the gas port is .006" bigger than the 10.5" and the round protrusion is .003" less than the 10.5". I will get pics tonight as there is no way I can explain what I did.
The only reliable way to measure is against the barrel’s torque shoulder or by comparing against a known standard like a headspace gauge.
Measuring from the rear face if the extension is not reliable as the length of barrel extensions can vary.
If you had a surface plate and a tall enough height gauge, you could work out the messurements from the front face of the extension.
This would be accurate enough to compare headspace dimensions as long as your bolts are roughly the same.
I have developed my own simple gauging tools that I use during chambering.
I broke away from work today at lunch to go shooting, but this time I took both Silver Bear and some old lacquer Wolf ammo. I also removed the extra power extractor spring, black bumper and o-ring from the bolt and replaced it with the original extractor spring and blue bumper. Well…I had success for awhile.
I ran through about 100 rounds of Silver Bear before the empty got stuck in the chamber. I tapped it out then ran close to 500 rounds of the old lacquer Wolf without a single problem! I wonder if the spring, bumper and o-ring had anything to do with the problem I was having. Granted, an empty still got stuck, but it did it after 100 rounds or so. Before, it was happening every 2nd or 3rd round. So maybe a combination of ammo and extra power extraction? I don’t know what to think now.
Weak ejector spring.
The extractor upgrades cause a problem when the ejector spring can’t override it.
It can also be a rim-thickness and extractor hook problem.
