Well I was doing load work up in my 20” BHW to see how it matched up to my Tromix and Bushmaster uppers. I was loading 275 TSX with Lilgun. First round was 36gr and 37gr, everything looked great, so I moved on. Went to 38gr and 39gr. Fired the 38gr loads and they looked great, nothing wrong, so I fired the 39gr and the second shot locked up the gun. I checked the first 39gr that was fired and it had heavy imprinting. Got home and mortared the rifle. Second 39gr brass looked worse then the first! First thing I did was pull the BCG, and sure enough, a lug cracked, and was slightly bent in. I checked my measurements on the 38gr loads, and they tracked right inline with what I was expecting to see. The 39gr charges had EXCESSIVE imprinting and case head growth. All brass from 36-39gr was once fired Hornady and all primers looked good, the primers from the 39gr charges were a little flattened, but nothing crazy. I think that had more to do with the bolt failure.
What I’m gathering here, is that the Underground Tactical bolt probably wasn’t heat treated correctly. This isn’t the first bolt from them I’ve seen have fail. Heard of a couple Grendel guys, too. This bolt only had about 100-125 rounds on it, most of which were factory ammo or handloads well within book, not showing even the closest sign of over pressure. So, I’m going to swap it out for my Tromix bolt, which has around 3 times as many firings, with a good portion of those pushing to find max loads, and still holding strong. Then I will start over and work up again, and see what happens. Posted a pic, hopefully you can make out the crack.