I must add a caution on this. You probably remember 6-8 years ago we had a long discussion where you helped me work over a Super Blackhawk "Hunter" that was giving me grief.
I did the extensive lapping program and eventually got the constriction out of the forcing cone area, but in the process I opened up the entire length of the bore by almost .002". This .44 now has .433 - .4335 throats, measures .432 at the forcing cone and tapers to .431 at the muzzle.
The only way it'll give any accuracy with jacketed bullets is to kick them in the butt so hard they slug up. The only bullet I can successfully do this with is Remington's 180 grain JHP which I buy in bulk from Midway. Over 29 grains of H110 with a WLP, it gives 2.5" 100 yard groups.
With cast bullets, I use a .433", 300 grain, GC WLN LBT bullet over 22.5 grains of W296 and CCI 350 primers. (In this case, the magnum primer makes a huge accuracy difference.) Groups run 1.5" at 50 yards with a scope or 2" with iron sights.
With conventional .429 to .430 jacketed or cast bullets, groups look more like shotgun patterns, 6" at 25 yards and 8" to a foot at 50 yards. Leading is horrific; copper fouling is noticible.
Before I you taught me about bullet fit, I did all kinds of things to try to reduce leading, using every bullet lube I could get my hands on and testing each with a significant suite of loads. As I recall, even with bad bullet fit, Rooster Red Zambini leaded least bad followed by Rooster HVR; I still use these today with bullets which fit well.
I fought that gun for over a year, then sold it. A year and a half later, after talking to you, reading some of Ross Seyfreid's writings on bullet sizing, reading some of LBT's literature, etc I *thought* I had the solution. When I went back to the gun shop, the "hunter" was still there so I bought it back, then ordered an LBT mold, called RCBS' custom die shop to get a proper sizer die for my lubamatic, got some RCBS gas checks and annealed them, and got after it.
Success came quickly. I got my buck with that gun that year. That gun is one of the hardest, most pain in the butt guns I ever owned ... perhaps that's why it's one of the few I've kept. Some day I'm gonna get an elk with that setup.
So, um, what was my point? Oh, that if you go down the road of extensive lapping to cure a constriction, you can simultaneously open up the whole length of the barrel. It may put you in a situation where you HAVE to use an oversized mold. You can also wind up opening up the bore to a diameter a hair bigger than the cylinder throats because the soft lapping bullets will slug up a little when they exit the cylinder and smack the forcing cone.
It's a good technique, and successful, but not without a couple pitfalls.