No muzzleloader bores are exact. There is no set standard that says.. .50 caliber rifles will be .500 in diameter. Some 50 calibers can range from .499 to .505.
Doc White discovered that the .504 bore was a good standard and produced his rifles to load slip fit. Slip fit means the conical bullets are not forced down the bore. Rather, you push it under the muzzle with your thumb and then sometimes the weight of the ramrod alone will almost set the conical. So his rifles run .504, .451, .410, and I think the .54 is a .540 diameter. When we order conical bullets for instance, I know my bore is .504 so I will order a .5045 diameter. This makes the bullet loose enough to slide down the bore and stay on the powder charge.
Now my Bison is listed a .504 but the bore is actually a .5035 because of a different barrel company made this barrel. So I order a .504 and they load perfect.
We shoot giant conical bullets.
And with the slip fit, we don't have to swab. We can shoot and they normally shoot like that group above. In fact I have stated that a White is a boring rifle to shoot. Every time I take it out, it shoots like that.
Sabotloader then came along and taught us that White Rifles shoot sabots just fine too. But he is the only one in the world that would put that plastic in a White...