I'd honestly be more concerned about my socks and scope than the headstamp. I haven't used a magnum for the last 15 or so elk, some of which are 2-3 times farther away than many people feel comfortable shooting and I've not been disappointed.
If you jump an elk and it has a chance to run, barring a CNS hit....it's gonna run. If you pop one while feeding, it'll likely just tip over in its tracks.....be it a 243 or a 375.