If you are shooting commercial ammo in a M1 Garand, I suppose it is possible that the gas mechanism is cycling so fast the case hasn't had time to "relax".
I'm not a Garand expert, but I've read that Garands and M14's (and their M1A cousins) are hard on brass because they are trying to extract the brass just a few milliseconds after the bullet has left the barrel. Pressures at the gas port are around 8000 psi, if I remember right.
The problem with typical .30-06 ammo is the use of slower powders, which develop more velocity by having a longer burn time. This doesn't make any difference in a bolt-action, but in a gas gun the "tail" of the pressure curve can drive the pressure at the gas port higher than the action was designed for.
And, if the chamber is rough, that may be the other contributing factor.
I'd try some military spec ammo and see if it behaves differently before changing springs, installing adjustable gas cylinder lock screws, etc.