I'm not a competitor but I learned about the gas thing when a 308 Ar-10 started pulling rims on the cases. It all still applies whether you're a hunter or a competitor.
To further explain, when a shell is fired, it swells tight against the chamber. If you have too much gas, the bolt will unlock and start to pull that case out before the pressure is low enough for the case to relax. The result is broken extractors and ruined cases.
Good thread.
