Might be a "machining problem" on the lower, then, if that hole isn't the proper depth. I don't have an unassembled lower on me right now, or I'd measure it for you - someone here has to have one, though. It's gonna be the exact same depth on a .308 AR as it is on an AR-15 lower, so just about any lower should do for measurement...
EDIT - Eff it, I've got a built lower that I'll strip down tonight... I'll get a measurement back in here before the clock strikes 12... I can take some additional pics, then, and show you how to get that thing together easier...