I think the best answer in a round ball gun as to why a .54 over a .50 is that a .50 won't do nothing a .54 won't do...

I have had some argue or mention that .50 balls are easier to come by...I haven't bought a round ball in over 25 years or so...Buy a mold and go to it, balls are easy to mold...I also buy pillow ticking for patches and make my own lube, so all I'm buying is powder and flints...

btw...The .45 I had, I sent to Ed Rayle back then, he turned it into a .40...Some of the most fun I've had is shooting squirrels with that .40 and 25grs of powder...Dang thing is cute when it goes off...I get 280 shots out of a pound of powder using 25 grains as well...