Guess the ard part is finding a ball/bullet that is a tad too big for the bore...the 45colt isn't a problem, as .457" is common enough and the bore/cylinder mouths shouldn't be larger than that.
The .45-70 may not be a problem if it is a new rifle as most are running close to the .457-.458" size....for an old rifle, it may be as large as .462".....the older the gun, the more variation in bore size.
So just how do you get a few bullets/ball of at least .462" to use as slugs? ANd if you find them, can you buy just 2 or 3?
When I don't want to buy a box of off-caliber sized lead bullets just to use one or two as slugs, I'll cheat. In your case, use a .45LC lad bullet (not a ball...a bullet). Put it in a bench vice and squeeze it...we don't care about accuracy, just that the diameter comes up to at least .462". It won't come up evenly across the bullet's diameter, but that's OK as all we need is a band to measure.
The minimum needed would be one for each chamber mouth of a revovler, and one for the bore of each gun.
Better is three for each bore of an OLD rifle. Prefer to measure just the breech end of the barrel just ahead of the chamber, the muzzle, and use the third to pass all the way through the bore looking for tight spots.
If no tight or loose spots in the barrel, the breech end is the one to pay attention to. With lead bullets, they are goind to become the diameter of the bore just ahead of the case when fired...so may as well make them .001" smaller than that size rather than letting gas-pressure size them up.
The easy way is to shoot an oversized bullet at very low speed and capture the bullet for measurement. This will only tell you the SMALLEST diameter that will pass ( if the barrel has a tight spot, it will make the bullet that diameter) but most folks think this is enough.
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