Hi Joe,
That's the thing, it does barely close the bolt with a field gauge (7.62x51 NATO Forster Field gauge, 1.6455", found here: NATO Chamber Headspace Gages - Forster Products), so it's unsafe to shoot normal ammo as it is right now. I guess the "easy" path is swap out locking shoulders (or use an L4 locking shoulder and hope it headspaces) and exclusively shoot .308 out of the gun. I would like to maintain the .303 option, hence maintaining the current locking shoulder and machining the L4 bolt somehow. Is it so hard that even carbide bits won't be able to scratch it? I figure just go very slow and methodical in shaving it down to fit, if I can, on the locking shoulder side as that's less material than the bolt face side.