best method for lapping forcing cone restriction

[bobke]

veral-
certain you can provide sage advice.  have about a two year old, stainless, convertible bisley 45c/45acp.  after the normal sorting out, testing, shooting with a variety of loads, etc, it went back to ruger for new cylinder, barrel, higher front sight.  all for very good reason.  came back with newer, truer cylinder(first one wouldn't range out on 3 chambers), new barrel, higher front sight not screwed in square to rear sight and a .009-.010 gap, so took it to a local gunsmith who set barrel back, set gap a just under .003, trued up front sight.  breech is square, cone cut 11d, lightly chamfered sharp edges, etc.  should shoot, except for the constriction at forcing cone where screwed into frame.  and a coke bottle shaped bore-fat in the middle, choked down a bit at the muzzle.

one opinion i've had is to send it back to ruger, as frame/threads are potential issue, not the barrel.  one installed in a tightly threaded frame-likely with worn tooling, is the barrel permanently set or is there springback if the thread fit is corrected?  depending on that answer, if permanent, would you hand lap the barrel only or firelap same.  i've already reamed the throats .4525, so really don't want to go much further with cylinder.  a soft lead slug yields .451, but not sure if it's from breech end or forcing cone, as it slides down the barrel's middle with little resistance. 

i'd actually rather install a straight barrel, true from one end to the other.  what are options for a high quality stainless barrel set up only for cast bullets of 260-300gr at 800-1200fps?  right now, i've only lightly lapped/polished barrel with a jag set up on a lewis lead remover that i can screw the brass nut down to put very tight pressure on cotton swatches swabbed with 800 grit lapping compound, with pretty good effect, but the loads i'm testing now with both 13bhn and 22 bhn bullets are leaving a strong lead trace at cone where restricted.  i'm kinda tired of sending this back to ruger for an uncertain fix, would just a soon try to lap it out without hitting the cylinder or have a new barrel installed.  what do you think?  thanks for wrestling through this rather long explanation, but haven't had any reasonable responses in other forums, as yet.

[Veral]

  Barrel constriction at the frame attachment threads is inevitable if the barrel is tightened firmly.  This because the threads are V shaped and apply a crush to the barrel when under high torque.

  Many people, like you have just stated, believe they can lap a barrel with a tight patch.  That cannot be done.  It will only polish over whatever humps are there.  The best method is to fire lap, which will make the barrel slightly choked by time you get the constriction out.  The barrel will be better than anything that any gunsmith can screw in, because, no matter how straight the barrel, it is going to choke.  Further more, any machining on the outside of a barrel, changes the inside, so, stamping, sight installation, etc, etc, all effect the bore.  Fire lapping is done after everything else, and makes things as near perfect as perfect can be.

  I've covered this subject several times in several ways on this forum, but it is very important and a good subject to hash over, again and again.

[bobke]

was pretty certain i hadn't positively affected a reduction in forcing cone obstruction by lapping with a tight patch.  would you hand lap the constriction with a slug til largely gone and finish it off with some level of fire lapping?  have the cylinder just about where i want it and don't want to go much larger in throats, if possible, with excessive firelapping.  i tried about 80 rounds of progressively lighter (200-1200) compound on gun's first barrel-prior to replacement by ruger, and the restriction in it's cone never went away.  if a hand lap of the cone with a soft slug could affect the necessary change in constriction without working the remainder of bore too hard, would that sound reasonable?  seems there might be some minor, positive carryover from just dragging the patch in/out, follow with careful firelap and see where it goes.  don't want to trash the barrel, wear lands/grooves or limit it's potential by overlapping, so want to proceed with caution.  just need some guidance before moving into the process.  have been buying molds from you since the cornville, az days-you've made a helluva mark on the history/science of cast bullet shooting.  thanks for that.

[EdK]

Is casting a lap in the barrel a satisfactory(or possibly superior) alternative to fire lapping but just beyond the ability of 99% of the shooters/ end users? I believe fire lapping to be beneficial however I still cannot understand why the added wear to the throat/forcing cone is not detrimental and to be avoided if at all possible. It would seem the abrasive laden lap bullets would be most effective when hitting the forcing cone and progressively less so toward the muzzle - hence the desirable choked barrel effect.

I've often thought about initially chambering a barrel in a small cartridge for a given bore (e.g. 357mag in a TC). Fire lapping, and then rechambering to the final cartridge (e.g. 35 Rem) cutting out the eroded throat from fire lapping in the process of the rechamber.

Wondering if the same could be accomplished by initially cutting some sort of perhaps less than ideal forcing cone which would leave too much meat but at least be functional to the point of getting the bullet guided into the bore, lap out the restriction(s), then re-cut an ideal forcing cone.

[jbquack1]

I have 2 rugers that had the same problems and used verals lapping kit to clean the bore's of both guns up.My problems were more in depth than the bores,the cylinders on both guns had chambers that were inconsistant both diametrically and depth wise.firelapping works well and as i said in a thread on this forum, i would recomend verals product over anybody's.He helped me alot the first time i did this.jim

[Veral]

  Don't even think about hand lapping.  It is HARD work and takes a tuned machinest mind to get proper results, which will be inferior to fire lapping.  Don't mess with other compounds, just use what I supply in the lap kit, just as the instructions explain. 
  Yes there is some wear in the throat at the rifling origin, and both cast and jacketed bullets benefit from it
  I formulated the compound in my lap kit, using multiple grits, because it cuts fastest and smoothest with the least throat erosion. (By multiple grits, I mean the compound has more than 20 grits, and I don't tell anyone what they are.)  Optimum bullet hardness for your lead lap bullets is 12 bhn, and this is important in minimizing throat wear.
  Cylinder throats get 1/6 as much wear as the barrel, so they rarely increase in diameter enough to make a difference.

  When I was fine tuning this process, way back when, I needed a revolver to lap with, experimentally.  I selected a S&W 38 and lapped on it till the barrel was opened up about .002 full length and was exactly the same diameter as the cylinder throats.  It was the most accurate revolver I have ever fired.