It sounds like some one did not clean the perseverative out of the barrel.
Suggest scrubbing the barrel with a brush and a solvent.
Swabbing the barrel with the other subs like 777 is necessary with Blackhorn it is not only not necessary but in all my inlines it causes accuracy problems.
Get a drill to fit your BP and clean out the carbon every 10 shots by hand.
Changing to mag primers will only help if you have a poorly designed BP.