Three of my S&W revolvers have barrels which are seriously constricted at the breech end due to compression at the barrel threads. This is evident visually as a faint ring shadow when examining the bore with a bright back light, and by greatly increased felt resistance as a slug is pushed through the last ¾ inch of the breech end.
After getting cylinder throats in compliance with groove diameters, and obtaining correct mould and sizing die relationships, this is the last modification needed to finally get these revolvers optimized for cast bullet shooting.
Lapping out .001 or so from a ¾” length of the bore seems tricky. Fire lapping seems too good to be true. Anyone out there actually done this so that just the tight spot is fixed? What’s the best compound to use? Thanks.

"Fire lapping" is a last ditch, desparate effort to make a bad bore shootable. It is usually ruinous to anything resembling accuracy in a rifle barrel. Think about it: who in their right mind would shoot an abrasive-charged lead bullet through a decent barrel?

Hand lapping, on the other hand, is the solution for barrels with tight spots or restrictions in the bore. Hand lapping gives you the control over a tricky process. Many custom accuracy barrels are hand lapped. The similarities between custom and corrective gunsmith lapping are minimal. They both require lead cast-in plugs, they both use progressively finer grits, require muzzle/crown protection, and attention to detail.

I've used automotive "Clover" brand valve lapping abrasives in the finer grits. You need a good, soid one-piece cleaning rod with a ball bearing handle. Various gunsmithing books describe the techniques used: filing or turning grooves on the end of the cleaning rod to retain the cast lead plug; pre-heating the rod and barrel before casting the soft lead plug; partially withdrawing the plug and charging with oil and fine valve grinding compound, and using repeated careful strokes in the area of the contriction. Remove plug, clean bore, and slug to determine uniformity of work. Repeat as necessary, keeping in mind that it is easy to lap the throat oversized.

Taylor throating was originally designed to cure your problem. Brownells sells the reamer which is used with the barrel in the frame, so no disassembly is required. You can read about it here:
http://www.alphaprecisioninc.com/revolver/default.htm

WOB