Thoughts on this new bolt? HM Defense

[ABNAK]

I'd rather see radiuses behind the lugs like LMT, an even smaller diameter cam pin, and stick with C158 for the steel.

I believe it is a smaller diameter.

[Ned Christiansen]

Cam pin diameter is the same.

[Hulkstr8]

I emailed them over at HM and asked why they use the bolt steel they do, instead of 158 Carpenter for example. The response was that they do in fact make their bolts out of 158 Carpenter sometimes, but it is harder for them to acquire; they also find that the 9310 is a good material a lot of other makers use so they do too -and it's readily available.

[BufordTJustice]

I emailed them over at HM and asked why they use the bolt steel they do, instead of 158 Carpenter for example. The response was that they do in fact make their bolts out of 158 Carpenter sometimes, but it is harder for them to acquire; they also find that the 9310 is a good material a lot of other makers use so they do too -and it's readily available.

What they're saying is true because C158 that is certified from Carpenter is only available in large quantities and is very expensive. It also takes slightly longer to machine.

9310 being a "good material" is not in question. But is it the BEST material, commonly available for an AR15 bolt and that answer is NO.


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[MSW]

Am I being OCD? It doesn’t say MPI/HP tested & neither do their barrels, unless I missed it on their website.

I’m not the most worldly shooter, but the only bolt breakage i’ve seen exactly once was a bolt lug break off & I don’t recall the brand.

For my money, i’d rather know it’s been HP/MPI.

[pinzgauer]

Bill Alexander, who knows quite a bit about building strong bolts, has indicated the real issue with C 158 is that it is tedious to get heat treated correctly for things as small as a bolt. The alloy was designed originally for much larger parts.

Thus the need for exact TDP on heat treating, and still the requirement for HP / MPI testing to catch the cases where the heat treat is not correct. Batch testing is not good enough.

He indicates that properly heat-treated, 9310 can approach c158 with correct heat treat and will beat c158 with inconsistent heat treat. Not perfect, but can be as good, especially if you want something that is not pre-stressed via HPT. (And HPT does prestress it, shortening life).

Clearly, there is not a magic alloy yet, though folks are still exploring.

Lately I just go with quality c158 bolts for 556, from a manufacturer that I know has QC to catch a bad ad heatreat. (Colt, DD, etc). LMT has the quality, but I believe they've started using other alloys. By all reports there are a handful of other manufacturers that could be included in that list. But I don't have personal experience with them. (BCM & Sionics come to mind)

On calibers like Grendel that put more stress on the bolt ( in particular bolt thrust, due to the larger base case diameter), I stick with the manufacturer that has literally fired Millions of Grendel through their products, and I know will get it right. Which for Grendel means Alexander Arms. Time will tell if they keep that level of quality with Bill moved on to HK Development.

I would go with LMT for 6.8 or 7.62x39 for the same reason.