06-02-2021, 03:00 AM
As VASCAR2 noted, the rim on the 7.62x39 and Grendel cases is thicker than on .223, so that is a big reason that Bill Alexander deepened the bolt face a little. It allows more meat on the extractor. As near as I can tell, early 7.62x39 AR's made as few changes to the standard AR15 as possible, so they kept the standard bolt face depth. For a long time, 7.62x39 AR's were not common, so there was not a big effort to optimize their bolts. By the time 7.62x39 AR's became popular, the 0.125" deep bolt face was the "standard" and it looks like noone wanted to buck the trend. They mostly shot factory ammo anyway, so there was not a lot of people handloading higher-pressure rounds. It is important to note that when Colt made a 7.62x39 version of the AR (CK901) for Yemen's military, however, they used an oversized bolt and extension for durability.
When Bill Alexander developed the Grendel, he was trying to squeak out maximum performance from the AR platform without changing the bolt diameter and barrel extension. This led to optimizing the bolt over what was already available. If I remember correctly, one improvement that Bill Alexander made on his Grendel bolts was to increase the radius of the transition from the bolt face to the lugs (inside the bolt). He mentioned that there were others improvements, but I don't recall specifics. There are various other improvements that have been made to AR-type bolts that are not Grendel-specific. One of the most common is to increase the radius of the fillets at the base of the lugs.
I know that KAK has done a lot of work on bolts but I don't know how much of that has carried over to their 7.62x39 variant. Heck, their E3 bolt for AR's is sufficiently different that it requires its own special barrel extension.
When Bill Alexander developed the Grendel, he was trying to squeak out maximum performance from the AR platform without changing the bolt diameter and barrel extension. This led to optimizing the bolt over what was already available. If I remember correctly, one improvement that Bill Alexander made on his Grendel bolts was to increase the radius of the transition from the bolt face to the lugs (inside the bolt). He mentioned that there were others improvements, but I don't recall specifics. There are various other improvements that have been made to AR-type bolts that are not Grendel-specific. One of the most common is to increase the radius of the fillets at the base of the lugs.
I know that KAK has done a lot of work on bolts but I don't know how much of that has carried over to their 7.62x39 variant. Heck, their E3 bolt for AR's is sufficiently different that it requires its own special barrel extension.