Bolt Group Buy - Possible?

[Tedward]

Nobody should be loading for the Grendel in the AR15 past the maximum published loads. 62,000psi is about 10,000 psi more than max loads. The bolt thrust and bolt geometry of the AR15 bolt barely support 62,000psi pressures that long in 5.56 NATO.

The original AR15 bolt geometry was built around a 50,000psi SAAMI max cartridge, the .222 Remington. When that evolved into the 5.56x45 (and civvy version .223 Rem), the pressures were raised significantly, without what I feel should have generated an engineering and blueprint change for the whole system. Since it would have only changed dimensions by maybe 2/10's of an inch, it wasn't pursued. (new bolt, new extension, new magazine, new upper)

Carpenter 158 was specifically selected for the military bolts because it could handle the higher pressures better than 8620, but they also added the High Pressure Test load, followed by Magnetic Particle Inspection (HPT/MPI) to see if microscopic stress fractures had been induced by the HPT load.

Carpenter 158 is very difficult for a shop to get, as you have to order high volumes of it. It is an excellent steel for AR15 bolts. Harder does not mean stronger, especially when you look at shock force on a steel part. You want a balance of substrate ductility, with great surface hardness, and a steel that takes heat-treating well without the surface becoming too brittle. If it is so hard that it fractures on the surface, you will start cracking bolt lugs or the cam pin hole walls.

Frank is very aware of these things, and has been dealing with AR15 BCG's as an OEM supplier for a long time. In the end, engineers will most likely be involved with selecting the optimum bolt material, dimensions, and heat treating, as well as extractor steel.

So are you recommending he sticks with Carpenter 158? He said he will use whatever we want. Mark from Precision Firearms is one of the people who said the 9310 is what needs to be used. I have no clue and am just a messenger, so if someone wants Carpenter 158 rather than 9310, they need speak up now.

As for the 10% increase to handle the hot hand loaded overload "NOT RECOMMENDED" accidental spike, Mark at P.F. said that the bolt should be able to handle the 62,000psi so it doesn't blow up.

I take it like a tachometer on a car, if it red lines and is not recommended, so it it doesn't blow up. Maybe a bad comparison but same safety buffer to protect the product.

Again, are you saying to use Carpenter 158 not 9310? If so, talk to Mark and see why he feels 9310 is what others are using like LBC. Not saying they are, just what is being passed along and Frank said maybe they were using 9310 because they can't get Carpenter 158 but he will use what ever we want...

Make the call so he can order the materials, I'm sending the bolt.

Thanks .