Almost all custom barrel mftrs will warn against lapping barrels AFTER they have been crowned and chambered. All you are going to do is open up both ends of the barrel and worsen accuracy.
Some barrel mftrs are also require you to relap the barrel after fluting as that will leave high spots in the bore.
As for the coppering. Two things to try Wipeout (foam that sets in the bore), check BR central for advertisers
sells the Montana Extreme Copper Cream. Works great but requires the use of hard bristle brush. Sinclari also sells. I use the Kleen Bore brush, has the hardest bristles I have found.
pressure variations will come from three areas.
1. different bearing surface lengths
2. difference in actual bullet diameters or pressure rings for each bullet
3. work hardening of necks