Since I'm not shooting a succession of shots when hunting deer, this is not my field loading procedure.
I understand the idea at the range for folks shooting competition for score in a match that uses scoring rings vs. hit/miss targets (such as steel). I know the practice was also the norm for the folks shooting paper patched conical bullet at 40-rod matches [220 yards], where slight variations really do translate into scoring differences.
Since my first shot from my barrel at a deer will be from a clean barrel, then my first shot at the target at the range should be the same, if I'm checking my sights.
It would cause serious problems for the folks who load at the beginning of the season and keep the rifle loaded until they harvest a deer or the season ends for them...., as the residual ash in the barrel in between the ball and the muzzle would slowly, over the hunting season, likely cause corrosion. IF they ran a damp patch down to the loaded ball and back, knowing the grease on the patch surrounding the ball would prevent the powder from becoming damp, followed by a dry patch, ..., doing this when they went out in the morning and when they returned at night.... then they are shooting from a "clean" barrel probably after returning from the second day of hunting.
LD