Just something I came up with today in order to find barrel size since I don't don't have any soft lead slugs to use. For me, this is easier than what I've gone through before to create slugs to use. If you have readily available slugs to use, by all means, use those. Please don't think I'm trying to get you to switch to this method.
This is easier for me, YMMV.
In short, make up a round with the bare minimum charge required for the bullet to clear the barrel when fired. Unfortunately, you'll probably have to clear a squib or two when you go too low. Once you get that "just barely" load, go get a roll of toilet paper and shoot into that. As long as your charge is low enough, the bullet will stop somewhere in the roll. All you have to do is search through the plies until you find your perfectly intact bullet.
Am I missing something that would cause the recovered bullet to not be just as good as a slug done in the usual way?