They perform better at longer ranges up in the thin air, but I'm pretty sure (not 100%) their lifespan is either tied to a timer or a range limit. I think it's the range limit. So while you might get a stronger "punch" from 50cals when shooting something 1k out at 25K, the bullets themselves don't last much more than 1.2 (or whatever the end limit is).
Just means they behave different until they expire, up in the thinner air.