Olympic ammo from Sportsman's Guide

I got home from lunch and found a case of it sitting on my porch. So the first thing I did was open a box and put on my hearing protectors. The ammo is advertised as being identical to the Nato loading, with a 124-grain FMJ moving at 1,251 fps. The rounds look good, with brass cases and crimped primer pockets. They're advertised as Boxer primed, though my press doesn't like primer crimps, and I'm too lazy to remove them.

I fired 24 rounds through two magazines loaded 13 and 11 so that I'd have exactly two full mags left. Feeding was fine, and the mags locked open on the last round fired through my satin chrome Mark III with the 18.5 recoil spring installed. One round did not fire, but cocking the hammer for a second try made it go bang. Everything worked well--until I picked up the brass, that is.

Every single casing showed completely flattened primers, and one primer was perforated very cleanly, with evidence of gas leakage. What I'm wondering is whether the flattening is attributable to soft primers and high pressures or to high pressures alone. As I said, functioning was 100% except for that one that needed a double strike.

Anyone used this stuff before? I'm stuck with the other 976 rounds of it; I bought it simply as a means of practicing with +P ammo without the +P cost, and I hope I can use it for its original purpose.