I took my carbine out to do some shooting this weekend for the first time in a few years and the bolt broke. The rifle was shooting fine for about 40 rnds of slow fire before the bolt failure. The bolt broke at the base of the lobe that connects to the op rod. This is the second time this has happened with this rifle. The bolt is USGI. The ammo I am shooting is wolf steel case that I bought from the CMP. I am not sure what's going on, any advice would be great? Thanks.