Stan,
I wouldn't be surprised to learn that some gunsmiths used a punch only as wide as the slot (making a bulge within the slot), but the ones I learned from were of the "school" that used a punch about as wide as the whole iron on the breach side of the slot. This had the effect of stretching the iron, moving the slot itself toward muzzle. Keep in mind that only a few thousandths of an inch movement are necessary.
Mike