Originally Posted by
DougGuy
If you're doing it right, the forging has no stress against it really, the stress is applied to the locking lugs and to the barrel nut, the upper is just along for the ride.
If you're doing it wrong, clamping the upper in the vise and twisting on the barrel nut, you will learn abruptly that the only thing holding the barrel in alignment in the upper is one tiny little pin, and if it turns it won't shear off, it rips a gob of aluminum loose and carries it with it as it turns, the upper is toast at this point.