Is the Big Fifty a good caliber for a round ball rifle? Seems like a decent go between from a squirrel gun and a big bore.
Yes, but it's not "between" in an "either or" sense, with a squirrel gun and a big bore (imho)
Folks don't really agree on what is meant by "big bore" either. Some say .58 or better, some say .62 or larger, and one chap that I've read who dealt with dangerous large game thought that .69 was for deer type animals, and .72 [12 bore] or larger was a "big bore" for things like tigers, or rhino, or elephant (well he used 4-bore for elephant).
A .50 tends to be quite hefty on impact, and for anything smaller than deer, it's pretty much head shots only.
Something .40-.45 with a round ball is more like the caliber that transitions for both small game and and big game, but even that's different, depending on the person whom you ask. For example, I have a .40, which I think is dandy for small game AND for deer (at the distances that I hunt). It's legal where I live for deer. A few folks live in areas that allow a rifle smaller, and have taken deer with a .38 or even a .36 at a close distance
Other folks live where the .45 is the smallest allowed for deer, but they have a very accurate rifle so do head shots on squirrel and rabbit, and some even on turkey (in the fall).
So..., it depends on what you want to do, really.
A majority of folks will say, I venture to guess, that if it's big game but smaller than an elk, .50 round ball is great. Some will tell you an elk needs at least a .54 BUT..., others will say they've taken elk with a .50 without a problem, and others will tell you No, and that an elk is much better when taken with a .58. Some of the members here love their .62 rifles, and wouldn't think of taking anything big with something smaller, and some will tell you YES a .62 but in smoothbore, so you use a round ball on the deer, and shot on smaller stuff...
Get something that is accurate, AND that you like to shoot, AND that you also like to carry. Then figure out the best way to use that for the shooting that you are doing.
To sum it up...
"There's more than one way to skin a rabbit", as the saying goes....
LD