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6 ARC bolt face depth
- Thread starter memilanuk
- Start date
I thought ARC had standardized on .136.
I always wished that all Grendel family cartridges had just went with .125 for the increased bolt lug support, but no one asked me..
I always wished that all Grendel family cartridges had just went with .125 for the increased bolt lug support, but no one asked me..
Not enough extractor lip thickness when you go .125”, which is why the deeper bolt face is a thing.
Colt did the engineering on all this in the 1980s for the funded 7.62x39 project. That became the basis for the .50 Beowulf and later 6.5 Grendel bolts.
They found that a .125” normal face depth didn’t allow a thick enough extractor lip for the x39 case rim depth and width, if they were going to keep the existing bolt lug placement and extension tooth geometry. You can have .125” face depth with Large Frame AR-10s because the extractor is so much wider, which provides plenty of material to support the lip (although my DPMS LR-308 folded the lip back within the first box of ammo). Metallurgy really matters on forming extractors no matter the cartridge or dimensions.
By adding larger radii to the face-to-inner wall angles, and to the rear of the lug-bolt shaft junctions, you can re-gain the strength lost from the deeper face. Combine that with certain aerospace-grade alloys in the 9000 series and you have great bolt durability. The new Colt 7.62x39s have a circumferential trench around the bolt face, which is another approach that I hadn’t seen before.
If you look at guys who shoot a lot of 7.62x39, they tend to go through extractors like nobody’s business, even with the LMT enhanced bolt with lobster tail. It still used a .125” face when it should have gone .136”, with corresponding barrels chambered for that to headspace correctly.
That’s very, very lucky because they were one of the main culprits in broken bolts, not just extractors.
Model 1 Sales was known for being gun-show grade garbage with very poor metallurgy.
People tried using their bolts for 6.5 Grendel, and I think they even had their imitation of Grendel called 6.5 Sporter. Those broke bolts left and right. The bolts didn’t last long enough to rip the extractor lip off.
Was your Mod 1 Sales 7.62x39 a 20” by chance?
Them being considered garbage was why I mentioned the name. Ya yit was 20" rifle gas. I broke some else's bolt in a 16" must have been mid because I don't have any carbine tubes around. I sold the barrel and got out of 7.62x39 at that time. I always assumed it just a junk bolt and it might have been. I remember the manufacture or reseller of bolt was out business when it broke.
That 20” RLGS in 7.62x39 has really low port pressure compared to a 5.56 even, so the ports needed to be large to deliver sufficient gas back into the expansion chamber in the carrier. It made for a mild gun on bolts in the 20” 7.62x39s.
16” were CLGS from Colt, Mod 1 Sales, and anyone else who tried to jump on x39 without doing due diligence in engineering. There was a lot more work done on the Colt guns back in the late 1980s-early 1990s, followed by the KAC SR-47. ArmaLite made a 7.62x39 AR-15 back in the day that was MLGS, which is pretty rare in 7.62x39.
