Just picked up my XCR. Haven't shot it yet, but hoping to get to that this weekend. While I was working on it this evening, I noticed that the upper and lower receiver are not quite aligned at the rear. The upper protrudes on one side, relative to the lower, and is inset on the other. I thumbed the lock and took a peek, and the blind hole at the back of the lower, where the end of the receiver lock rests, appears to be a bit off-center. Is this normal/correct/ok? Is there a tolerance or spec for the position of this hole?