Bob, I've seen that same thing happen with .270's, .308's as well as .30-06 and 8x57 Mauser rounds. Each and every time was caused by not enough powder in them; I had reduced my starting load too much.
I've also seen it happen with factory and military ammo loaded to normal max safe pressures but fired in an M1909 Argentine Mauser whose barrel had been reamed with a .30-06 chamber. Its .303 bore and .312 bore diameter didn't cause enough pressure for the .308" diameter .30-06 bullets.
While your barrel may be OK headspace wise, I'd have it checked for excessive throat erosion and bore wear which can cause the problem with factory ammo.