For what it's worth, the .58 Minies that I buy from Dixie for my original 1862 Springfield fit a little tight - they require a bit of pressure most of the way down. Now, that could be because LG&Y made the barrel a little small or just that, after 150 years, it's just not in perfect condition anymore
I suspect that, other than my great great grandfather and me, nobody really cleaned it the way that it was supposed to be cleaned after shooting it. Fortunately, other than a few years during the 1860s, it did not get shot much.
I shoot it so rarely that I honestly don't know what the diameter of the bullets is - I bought 50 of them a few years back and probably have 30 left. But I
think that they're .575s.