Our 3 lug mounts are 17-4 and our 3 lug rear cap assemblies are 17-4. Both are nitrided, rated for 5.56mm and 300BLK supersonic, I've run them on .308. No problems they were designed to be durable and centerfire rifle rated. The springs in the mounts are 17-7 rated for use to 750F, so a little conservative on ROF - like 2-30 round mags full auto.