First, they made a hole.
It measured .303" diameter.
Then, they put grooves in it.
THAT made ANOTHER diameter that was bigger, namely anything between .311" and .316".
Not odd, just the way they measured stuff back then.
Same with .577" and the .600" bullet.
The 45-70 Govt with the 457" diameter bullet.
......and so on.
The average .44cal BP revolver shoots a ball anything from .451" to .457" diameter.
It all harks back the days of black powder and deep-groove rifling.
tac