you can lap a bore with valve lapping compound on tight fitting patches or you can load up real light loads and coat the bullets with valve lapping compound...or a few companies sell pre coated bullets for bore lapping...this should be done judiciously as you only want to smooth out the bore and not take off so much metal as to lose accuracy.
http://www.davidtubb.com/final-finish-tms
my Winchester ‘73 had extremely light rifling...I assume it was shot with corrosive rounds...put away dirty and then eventually shot again...the barrel was probably just a rusty hole...and when shot again it cleaned out the rust and most of the rifling....the chamber was also enlarged...
I slugged the bore by driving a larger lead bullet down the barrel with a rod and mallet and measured the bullet with a micrometer and what should have been .427”(the old .44 were .427” as oppose to modern .44s which are .430) the bore measured .432” at its tightest point...I cast some bullets out to .432 but they were not as accurate as store bought .431” bullets from Oregon Trail.
with the almost shot out bore I am able to group 3” to 4” at 100 yds....more than accurate enought for most uses that an enemic .44-40 should be used for out to 100yds...
at cowboy action shooting ranges it always rings steel...
and I poured thousands of smokeless rounds through my almost 130 year old piece.
good shooting.
Tubb makes lapping bullets, copper jacketed with abrasive impregnated in the jackets. You get 50 bullets, 10 each of five separate grits.
I bought some intending them for the Enfield, but since it shoots as it is, I used them to break in the barrel on an AR-10 with excellent results. Near-zero fouling after an entire day at the range and sub-moa accuracy with anything that says 308 (as long as the rounds have good concentricity, that is)!