Do the 7.62x39 AR15's have bolt head issues?

[rickt300]

Why has the 6.5 Grendel bolt head been improved and the 7.62x39 still uses the original design? Is there some reason the improvement is not needed for the Russian round?

[VASCAR2]

Colt used the .136 bolt face depth in their 7.62x39 AR-15’s. Other manufacturers used the .125” bolt face depth which was in use on the 5.56/223 AR-15’s even though the rim thickness was different. Colt used the .136” bolt face depth to provide more material for the extractor. Colt also lengthened the tail of the bolt .01” so the same firing pins could be used with 5.56/223 bolts.


I’d assume other 7.62x39 AR-15 manufacturers didn’t test their designs or assumed the .125” bolt face depth bolt was good enough.

[Red*Lion]

Check out KAK's AR offerings in 7.62x39. They design their components such as their BCG's to be exceptionally sturdy. You will not have bolt issues with KAK products. I get around 1.5 moa with my 11" AR in 7.62x39 using a KAK bcg and barrel shooting steel cased Russian ammo.

https://www.kakindustry.com/

[lazyengineer]

Lower P round. So more forgiving

[rickt300]

Lower P round. So more forgiving

Probably it, I have seen only one bolt ever broken and it was a 6.5 Grendel.

[StoneHendge]

The 7.62x39 bolts do go through extractors like Joe Biden goes through pudding cups.

[BluntForceTrauma]

From KAK's website for their 7.62x39 bolt: "THE EXTRACTOR IS CONSIDERED A WEAR ITEM AND IS NOT COVERED AFTER SHOOTING."

[Fess]

As VASCAR2 noted, the rim on the 7.62x39 and Grendel cases is thicker than on .223, so that is a big reason that Bill Alexander deepened the bolt face a little. It allows more meat on the extractor. As near as I can tell, early 7.62x39 AR's made as few changes to the standard AR15 as possible, so they kept the standard bolt face depth. For a long time, 7.62x39 AR's were not common, so there was not a big effort to optimize their bolts. By the time 7.62x39 AR's became popular, the 0.125" deep bolt face was the "standard" and it looks like noone wanted to buck the trend. They mostly shot factory ammo anyway, so there was not a lot of people handloading higher-pressure rounds. It is important to note that when Colt made a 7.62x39 version of the AR (CK901) for Yemen's military, however, they used an oversized bolt and extension for durability.

When Bill Alexander developed the Grendel, he was trying to squeak out maximum performance from the AR platform without changing the bolt diameter and barrel extension. This led to optimizing the bolt over what was already available. If I remember correctly, one improvement that Bill Alexander made on his Grendel bolts was to increase the radius of the transition from the bolt face to the lugs (inside the bolt). He mentioned that there were others improvements, but I don't recall specifics. There are various other improvements that have been made to AR-type bolts that are not Grendel-specific. One of the most common is to increase the radius of the fillets at the base of the lugs.

I know that KAK has done a lot of work on bolts but I don't know how much of that has carried over to their 7.62x39 variant. Heck, their E3 bolt for AR's is sufficiently different that it requires its own special barrel extension.

[BluntForceTrauma]

Fess, KAK is a different company than KAC.

[Fess]

Oops, I need to pay more attention.

[Red*Lion]

From KAK's website for their 7.62x39 bolt: "THE EXTRACTOR IS CONSIDERED A WEAR ITEM AND IS NOT COVERED AFTER SHOOTING."

Extractors are wear items on all AR calibers.

[LRRPF52]

Why has the 6.5 Grendel bolt head been improved and the 7.62x39 still uses the original design? Is there some reason the improvement is not needed for the Russian round?

Colt did full engineering analysis and testing on 7.62x39 in their AR-15s, which was well-funded.



Bill A. spent time tracking down one of those project engineers to see what insights they learned from the 7.62x39 AR-15 program when he was doing his due diligence for the .50 Beowulf (which uses a rebated 7.62x39 case head), and they did a lot of things differently.

A few Hi-power shooters immediately started taking Colt 7.62x39 Bolts and chambered their tube gun AR-15s in PPC-based wildcats.




The after-market just assumed that you could make AR-15 bolts with the wider bolt face for 7.62x39, ream them to 7.62x39, and crank out rifles and parts with a cheap magazine to support it.

Their understanding and familiarity with 7.62x39 was so lacking, many of them used .308” groove barrels, totally unaware that Russian 7.62mm is actually .311”.

The after-market imitation 7.62x39 AR-15 rifles and parts were such an abortion, you had all these combinations creating excess chamber pressure, bolt thrust, and bolt torsion with really weak extractor lips.



Broken bolts and extractors became the norm.



To do 7.62x39 correctly, you really need to start with a proper 6.5 Grendel bolt + extractor and have your headspace set-up based off of that.

The .125” face depth bolts are a recipe for extractor failure for the same reasons they are with any other .444” case head in the AR-15.

You also have to really manage the gas system into the window of where reliability and durability find a happy trade-off, which is hard to do with Russian steel case ammo and its inconsistencies.

The reason is that heavy-gassed operating systems cause early rotation of the bolt while under partial load, which is harsh on the lugs and the extractor.

But heavily-gassed operating systems have more reliable primary extraction and full retraction of the BCG with inconsistent charge weights.

[rickt300]

Interesting there LRRPF52, So the right thing to do is use a Grendel bolt. Seems it would be hard to find a barrel headspaced to match.

[LRRPF52]

Interesting there LRRPF52, So the right thing to do is use a Grendel bolt. Seems it would be hard to find a barrel headspaced to match.

If I was putting together a 7.62x39 AR-15 barrel and bolt, I would start with a JP Grendel bolt and have the barrel chambered to that.

Otherwise, invest in a lot of replacement extractors or entire bolt assemblies.

Another problem is the inconsistency of steel cased 7.62x39 ammunition.

AKs have so much slop and over-gas that they run really hard to deal with that (huge gas ports).

[chic]

So, if I understand correctly, use a Grendel bolt and cut 0.011" off the barrel?

[lazyengineer]

Colt did full engineering analysis and testing on 7.62x39 in their AR-15s, which was well-funded.



Bill A. spent time tracking down one of those project engineers to see what insights they learned from the 7.62x39 AR-15 program when he was doing his due diligence for the .50 Beowulf (which uses a rebated 7.62x39 case head), and they did a lot of things differently.

A few Hi-power shooters immediately started taking Colt 7.62x39 Bolts and chambered their tube gun AR-15s in PPC-based wildcats.




The after-market just assumed that you could make AR-15 bolts with the wider bolt face for 7.62x39, ream them to 7.62x39, and crank out rifles and parts with a cheap magazine to support it.

Their understanding and familiarity with 7.62x39 was so lacking, many of them used .308” groove barrels, totally unaware that Russian 7.62mm is actually .311”.

The after-market imitation 7.62x39 AR-15 rifles and parts were such an abortion, you had all these combinations creating excess chamber pressure, bolt thrust, and bolt torsion with really weak extractor lips.



Broken bolts and extractors became the norm.



To do 7.62x39 correctly, you really need to start with a proper 6.5 Grendel bolt + extractor and have your headspace set-up based off of that.

The .125” face depth bolts are a recipe for extractor failure for the same reasons they are with any other .444” case head in the AR-15.

You also have to really manage the gas system into the window of where reliability and durability find a happy trade-off, which is hard to do with Russian steel case ammo and its inconsistencies.

The reason is that heavy-gassed operating systems cause early rotation of the bolt while under partial load, which is harsh on the lugs and the extractor.

But heavily-gassed operating systems have more reliable primary extraction and full retraction of the BCG with inconsistent charge weights.

That is an awesome post!

[Jeepster18_88]

I’ve built quite a few and my first ones both have well over 1k rounds each. Not necessarily high round counts but I haven’t had a failure of a bolt or extractor on any of them. It’s a stronger bolt design than the Grendel bolts so the assumption that it’s going to break bolts often doesn’t even make sense. The extractor breaking is more realistic but again, I haven’t experienced it yet. It’s also interesting to assume that it’s lower pressure when most AR 7.62x39 barrels have a groove diameter of .308 and they’re swaging down .310-.311 bullets so it’s quite possible for the pressure to be as high or higher than Grendel loads. It would be interesting to know how much that raises the pressure. In reality the cartridge should be able to handle the same pressure as the Grendel.

[VASCAR2]

Another issue with bolt life in the 6.5 Grendel is the amount of gas in relation to the gas port size and barrel length. AK pattern long stroke gas piston rifles are known for having excess gas. In an AR-15 excess gas can contribute to early bolt unlocking or the bolt trying to open before pressure has dropped. If the timing is off with the AR-15 it has a tendency to accelerate wear. Adding a suppressor can change the pressure curve in an AR-15 and might be mitigated by using an adjustable gas block, adjustable bolt carrier or adjustable carrier key.