Been reading your thread here and just wanted to offer a suggestion and link regarding fire lapping.
Beartooth bullets offers lapping compound online as well as full lapping kits including bullets. Also they
offer a great technical guide in book form. Its a small book easy to read but very informative regarding
slugging your barrel, fire lapping and do's and don'ts regarding all of this. They offer good info on why
to use certain bullets for fire lapping and why not to use certain bullets like bore riders. The lapping
compound they offer is silicon carbide grease mix in 320 grit which I think is the best grit if you are going
to use just one grit. I think the lapping compound is $17.00 the book is $15.00 and I think the entire kit
is $55.00 or so. http://beartoothbullets.com/index.htm