There was some really bad Koren surplus a few years back. Certain lots would split cases and cause all kinds of problems. Most is used up or broken down for components by now, but it occasionally shows up.
good reason not to shoot gunshow ammo.
M1s were designed for Ball M2 150gr FMJ for
2700 or so fps muzzle velocity. Get away from
these parameters and one is flirting with disaster
or at least a bent op rod.