Is the smoothie you acquired an antique gun?
First off make sure it’s not loaded.
Some antique guns have bores that are worn at the muzzle so you need to know what the bore is all the way down to the breach. If you try to force a .75 ball down that bore and it’s actually .70 at the breach it’s gonna get stuck.
Get a wood dowel that is .75 and see if it goes all the way down. If it does , a .75 patched ball will probably be a good choice or a bit bigger for bare ball.
If the dowel goes down easy you can do one wrap of tape then 2 wraps until you find some combination that is just snug.
Don’t force anything down the bore until you know the correct diameter.
Good luck and be safe