Headspace Issues? 0.125" bolt on .264 LBC doesn't close on go gauge

[LRRPF52]

Gotcha. So a .264LBC could take 0.125" or 0.136". Any idea how to find out? I looked on the upper and couldn't see any "Type I" vs "Type II" designation, which I think is what to look for. Also didn't see anything on the barrel but admittedly haven't taken off the freefloating handguard.

Yes. "Type I" and "Type II" are misnomers that should have never caught on with the 6.5 Grendel market (at least a decade after 6.5 Grendel had been introduced), but they did.

If the real Grendel chamber and bolt had been around all that time, but then another company for their internal purposes calls a .136" face depth "Type II" because they adopted it after using .125" face depth initially, it adds a lot of confusion to the market because people are looking for a simple, easy way of identifying 6.5 Grendel compatible versus after-market work-arounds that didn't work so well (.125").

Les Baer always chambered with real Grendel bolts.

Others that didn't know about the critical bolt dimension differences figured that using one of the poorly-executed "7.62x39" bolts would work just fine, so they chambered to those bolts with .125" face depth since they had a lead on sourcing those types of bolts. This was in the wake of popularity where several companies that never looked at what Colt did with the 7.62x39 Sporter figured they could easily chamber for 7.62x39 by simple specifying a bolt with a larger face, never mind rim thickness, extractor throw, extractor claw thickness, AR15 barrel extension Inner Diameter for bolt and extractor clearance or even bore diameter. A lot of companies initially chambered .308 groove barrels in 7.62x39 thinking Russian 7.62 and US 7.62 were one in the same. (Russian is .311", with a separate line of projectiles even supported here in the US and Europe).

The best way to see what chamber/bolt headspace you have is to measure your fired case headspace with the Hornady comparator kit. Your shoulder location with the B350 bushing (.350" datum location in SAAMI drawing for maximum cartridge) should be somewhere around 1.220" from case head to that shoulder location once fired, midway on the shoulder (.350" diameter).

Unfired cases/cartridges pretty consistently 1.210", and will grow .010" after firing. If your fired cases haven't grown much over 1.210", you probably need the .136" bolt. If they are growing to 1.220", then you have the correct bolt. The best .125" bolt is the 7.62x39 LMT Enhanced bolt made from AerMet. Even those have failures with extractor because 7.62x39 has such a thick rim, which doesn't allow you to get much longevity with a thin extractor lip.

You need to re-design the bolt in order to get a thicker extractor lip, or wider extractor lip. The AR15 barrel extension lug arrangement doesn't allow you to space the adjacent lugs to the extractor any farther apart for a wider extractor, unless you eliminate one of them like POF did on their AR15 in .308 Win.

This is why the .50 Beowulf and 6.5 Grendel have the .136" deeper bolt face, so that a thicker extractor lip can be utilized. It's also why Grendel bolts are longer at 2.810", so standard AR15 firing pins can be used for the most market commonality and minimal upset to parts sourcing. The extractor shelf location and pin locations are also different, to accommodate ease of articulation of the extractor over the cartridge and into the barrel extension.