Many years ago I was playing with the same idea. Took the last 2" of barrel and made a bullet sizer. Had to shrink it down a bit by turning it down on the OD and pressing it into a sleeve. The bullets that were sized and lubed had the rifling engraved so all you had to do was line up the lands and grooves and push the bullet down the barrel. Using soft lead the bullets did not need to upset very much to seal the bore. Loading was easy and they shot very good.