Agree with most of the above, but I never slug from the muzzles. I just bash a fishing sinker to approximate 10% oversize dimensions with a hammer, then drop it into the chamber throat and drive it into the oiled bore with a dowel. I like to push them right through, but with the likes of the Martini Henry with slightly tapered bore, the slug should probably be driven a few inches into the bore and then back out the same way.

Bore slugs from .360 to .577
ex-fishing sinkers all!

BTW, the new owner of a .470 in the first half of last century never slugged his bores. He just bought factory ammo or components over the counter, or through the colonial agency, and started shooting! No real reason not to do the same today.

There is a method for accurately measuring bore and groove diameter in barrels with odd-numbered rifling. It involves dropping the bore-slug into a right-angled (90-degree) 'cradle' in such a way that the lands of the slug are resting on the 'sides' of the cradle, and a land is uppermost. The distance between the 'apex' of the cradle and the top land is measured, and a formula used to convert this to barrel groove diameter. I'll try to locate the details. Meanwhile, anyone here know the formula, or an appropriate reference?