How does deepening a bolt face make it stronger?

[rickt300]

.136 vs .125? It would seem the shallower depth would be better.

[VASCAR2]

The deeper bolt face gives more room for the extractor and can contribute to longer extractor life. In my experience over gassed system and hot reloads are responsible for most 6.5 Grendel extractor/bolt failures. I’m not sure the .136 bolt face depth affects the bolt integrity any more or less than the .125 bolt as long as pressures are kept to 52,000 psi or less. Having a square receiver where the bolt squarely aligned with the barrel extension can have an effect on bolt life. Having a bolt/BCG not square with the barrel/barrel extension is a recipe for more wear.

[rickt300]

I have both bolts and though the difference of .011 is small the force is pushing the bolt lugs forward and it would seem the .136 depth would have less supporting the bolt lugs. The only extractor I have broken so far was on a .136 depth bolt.

[tashley680]

With the .136, aren't the bolt lugs .011 longer or am I missing something?

[Bill Alexander]

If taken as a square bottom recess the increased depth will remove material that supports the lugs in calculated shear. However the bolt design is a little more than simply boring out a 5.56 type bolt. Ignoring material and heat treatment correctly manufactured parts have changes to the lug root geometry as well as the radius at the bottom of the recess which is provisioned by the case design. Lugs themselves should be ground with a very slight angle to shift contact stress to the inner diameter.

Extractors should be austempered 4340 alloy with either a fully radiused back either side of the support flag or the surface raised by 0.011" compared to a 5.56 part. Conditioning is peened and burnished. Ideal is not phosphate and most definitely not chromed.

[LRRPF52]

Great to see you pipe-in Bill!

I think drawings would help people understand what you’re talking about.

[rickt300]

Another thought. Seems that if the bolt face is deeper that means the case is supported by the barrel .011 less. Are Grendel case heads solid further up the case to overcome this?

[LRRPF52]

Another thought. Seems that if the bolt face is deeper that means the case is supported by the barrel .011 less. Are Grendel case heads solid further up the case to overcome this?

As long as the feeding cone in the breech is cut correctly, you have full case support.