.54 is an advantage over .50 because of the heavier balls and bullets available. These blackpowder ML guns work more on mass of the projectile than velocity and the like that modern guns do. Blackpowder is a weaker propellant than smokeless, and the projectiles will travel slower than modern guns' projectiles. Heavier the ball or bullet, more likely it is to kill and kill quicker. A large wound is what you want with a ML gun. A .54 with a heavy conical bullet is quite capable of taking bears and moose and such. Round balls will work on most anything else. A .50 is just fine for most people but if by chance you might encounter a large bear or something, a .54 will be nicer to have. I've seen 600+ grain conicals available for the .54 though you'll have to search hard to find them, the ones I have (600 grains) were hand cast by someone with a mold who sells them. When I find a good price on a mold (and have the spare cash for it) buying them won't be an issue. The barrel will need to take a hefty charge but not too much (as a rule I don't like to go over 120 grains behind the large conicals I've used), the modern inlines' 150 grain charges is a waste of powder. .50 rifles are easier to find parts and such for, but the .54 offers some advantages that make it worth the extra effort to find parts I think. Round balls and low charges (I use around 70 grains most of the time but this varies with the rifle and you'll need to find an accurate charge for your individual gun) will work on many smaller animals and the large conicals make it suitable for very large animals. It's very versatile.
.58 is also quite useful, but, IMHO, it gets to the point of diminishing returns versus the cost. Used .54 guns are fairly easy to find and cheap, .58's not so, so considering the .54 is adequate for almost everything in North America, I don't think the .58 is worth the extra money, unless you're just interested in the guns that originally used it, like the guns of the Civil War...that's a perfectly good reason to buy one but if that's not the case I'd go with .54
I wouldn't worry about BP igniting on its own. Static electricity can set it off of course but then, it could set off the substitutes too. Just use common sense with it and you won't have any trouble. Good luck finding it though, I generally have to order it from Grafs (don't have the money for large 20 pound plus orders that some cheaper places want so I usually get 4 or 5 pounds at a time), no place around me stocks it most of the time, only pyrodex, which is okay in a percussion gun if you load it carefully (make sure to shake it a bit so it gets further down so it'll go off with a #11 cap) but terrible in a flintlock.
