IMO, cleaning up the muzzle/crown can improve it's grouping.
I've chamfered several of my revolvers & my unscientific observation is that some of them grouped consistently better afterwards.
Some had no apparent change.
If done properly it will not have a negative affect.
.
627-5 PC, barrel crown, before chamfering
(-03a)
.
627-5 PC, barrel crown, after chamfering (before polishing)
(-01a)
.
Brownells Chamfering Tools
(-01a)
.
Brownells Chamfering Tools
(-02a)
.