Daryl is correct.
You must remember that caliber was the same in the days of the muzzleloader as it is now.
It's only the diameter of the bore.
A thin barreled fowler would not be safe to fire with heavy charges and hard balls like the same caliber from a "planes rifle"

Both a 30 carbine and a 300 H&H are 30 cal, but that's where the similarity ends.

I hope I make the point.
It's oversimplified a bit, but it's a good place to start learning.