Thread: Rebore.

Originally Posted by akaroa1

The variable spring back on an existing profiled, bored and reamed barrel means that pulling a button through it would give a significant variation in the final bore and groove sizes.
The muzzle which is usually the thinnest section of the barrel would be bell mouthed, because it is thinnest there is less metal too spring it back.

Same effect as cutting a muzzle thread on a stressed barrel - reducing the diameter allows the metal in the muzzle to expand giving a bell mouth effect.