Well, I learned on a .50 Lyman Trade Rifle. So I have an affinity for that caliber. My wife liked it so I bought her one. The .50 is so versatile, and if you forget balls or patches on the way to the hunt or the range, someone else is likely to be carrying. Certainly kills a lot of deer here in PA, but can't be legally used for small game, so it does have some limitations. I probably would not use it for elk or moose or bear.
When I got more serious about BP, I wanted a custom gun. I decided on a .54 for several reasons. I pored over books looking for a style I liked. Couldn't get away from the Golden Age Lancaster/Lebanon guns for their beautiful simplicity. Finally settled on a JP Beck circa 1785 that my gunsmith made for me. Love it! Back then in the golden age, there was still plenty of big game in PA, so most long rifles were still larger caliber. (Of course, so many rifles were odd calibers, but that's beside the point.) So the .54 made sense from an historical perspective. Secondly, I like big slow bullets--terminal ballistics as I like to call them--in my opinion are the best bullets to anchor game, all else being equal. I'm not a very good hunting shooter (yet!!!) with my .54, so if/when I don't put the best shot on a deer--for whatever reason--that extra energy from the .54 will go further to putting the deer down than if I had made the same potentially poor shot with a .50 or .45. [Of course, one might argue that recoil with a .50 or .45 would be less leading to greater accuracy...] Lastly, after consulting with my gunsmith and barrel maker (LC Rice), I learned that the ballistics of .54 were a little superior to the .50. And I like accuracy. Finally, should I ever get the chance to hunt elk or moose or bear, I feel confident the .54 will work with a 231-grain round ball. (Might even try some two-ball loads!?!?!). My twist in the .54 is 1:72, so I can never shoot anything but PRBs as the slow twist won't stabilize a conical.
Now, I'm having a small game rifle made. It'll be based on a Thomas Allisson, western PA caplock. Those western PA guns, circa 1800-1840 were made at a time when most big game in PA was extirpated or in severely short supply, so the originals were smaller calibers. The Allisson original is .35, so I had hoped for a .36 cal swamped from LC Rice. No luck. Says he can't get his .36 cals to shoot very well, so I opted for a .32, which he said shoot very well, if you don't mind swabbing the bore often while at the range. Didn't want to go all the way to a .45 (.44 is minimum legal muzzleloader cal for deer in PA), so, figuring this Allisson will be 99% for squirrels, the .32 was the choice.
I can't bring myself to get excited about larger calibers (.58, .62, etc.). I'm building my first longgun this winter and will be dropping a .45 in. I like that compromise as that's the closest thing to the average between my .54 and soon-to-be .32. It'll make a nice smokepole for my first child, who is due in July.
I've rambled on long enough....
VF