A friend and I both picked up 5.5" Buck Marks on sale when the local gun shop closed. I've only shot standard velocity stuff, but my friend has shot everything from hi-velocity down to primer-only rounds. According to him, the primer-only stuff (no powder in the case) does not cycle, the subsonic stuff he's tried (doesn't remember brand) definitely had a high failure rate (as many as 2 per magazine), and standard ammo (as expected) works every time.