I think you're on to something there. Same bullet (120, lighter charge by two full grains will lock it back. In theory, with a charge two full grains higher, you should be producing quite a bit more power, speed, and gas pressure. The rifle should cycle even harder, have more felt recoil, and lock-back on last round should be easy like that.
But, it's not. Because that extra two full grains of pressure isn't getting there, through a small gas port that won't allow it. Instead, the bullet is gone from the barrel, and pressures drop.
Pull the gas block and measure that gas port. I don't know schit about .260 Rem gas port sizes, but I'm curious to see what it is anyway.