I have a Daniel Defense that I wanted to put an adjustable gas block on. After removing the stock item and then comparing to the aftermarket adjustable block it look as if the gas hole on the new one is just a hair different from the stock unit in distance from the shoulder.

I would assume all barrels drill the hole to the same spec from the shoulder.

Or is my above assumption wrong? Do people ever have to shim these? I would like to not shim such a critical item.

I think it depends on the barrel pattern or the block pattern or something. I recall specifically reading about that hole being different on gas blocks or barrels depending on whatever the rifle is patterned with. On my 308, I slam that block against the shoulder, but I was reading somewhere that there is supposed to be a gap on some of them for some reason or they will not line up.

I don't think the assumption is correct completely.....I'll let others chime in but I do recall hearing about this 'issue'.