"Naked" bullets loaded in a ML are never "tight". They are sized to the bore when loaded.
Cloth patched balls or picket bullets are far harder to move since the compressed fabric grips the bore very well.
I would never use a Maxi or REAL etc in a double rifle.
Then we need to realize that the bullet can likely be 2" off the powder with no damage to the gun though accuracy will suffer. But 6" could be bad.
So do some testing. See what actually occurs in YOUR situation.

Dan