When you lap a barrel, the lap is cast in the bore and is a close fit with the corners of the lands and grooves and makes full contact with the grooves.
When you use JB Bore Paste, the patch doesn't make perfect contact and would probably round off the sharp top edges of the lands before it lapped everything else. This would probably take years of use.
JB will help shine up a bore or keep it shiny but it's not intended to be a lapping compound.
A better option would probably be to use one of the fire-lapping kits or use one of the break-in processes of shooting a round and cleaning for a certain number of rounds.