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Beartooth Regular
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Discussion Starter #1
Hello everyone,
     Began the process of firelapping the SBH today and would like to pass on my experiences. The load I used was 2.7 grs of AA #2 Imp with Marshall's lapping bullets. I began with 20 rounds using 220 grit. Incidentally, I am using the compound from the Midway bore lapping kit. I contacted them and after a LONG time they got back to me and said the abrasive used is the same type as in Clover valve compound. Since I already had the kit I from a previous rifle lapping job I used it. Anyway, the first twenty rounds were with the coarsest grit, from here out I'll use the 320 grit. I seated the bullets like a wadcutter, no nose showing.
     Some observations- I seated the bullets in unsized cases per the technical guide, and because of their large diameter they are a TIGHT fit in the chambers.
    I got no bore leading but I did get some in the forcing cone and chamber mouths. I cleaned the revolver between every cylinder full. The lead in the chamber mouths was easy to remove and less build-up was noticeable with each cylinder full but the forcing cone area is HARD to get clean completely.
     My group was large and higher than where my usual loads hit, confirming that lower velocity = higher impact point ( I had to see it myself!!), but I noticed the last cylinder full seemed to add a lot of new holes to the center of the cluster.
     Questions: Do I seem to be doing this right? Any suggestions? How about cleaning- do I need to be so meticulous with the forcing cone or can I assume (dangerous!) that the lapping bullets are polishing out the part that needs to be polished? Marshall, or anyone with experience, I would appreciate your input!
                   Have fun and shoot straight,  :biggrin: ID
 

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The Hog Whisperer (Administrator)
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Sounds like you are doing a great job so far.

Can't have too much cleaning - my opinion.  Don't want to let that grit build up and scratch anything that it shouldn't, so much better to be prudent and clean every cylinderful, as you are doing.

As far as the forcing cone - didn't experience this with my lapping jobs so can't comment too much.  Hopefully the more experienced can step in and address this topic.  But - I cannot for the life of me see how cleaning it thoroughly every 6 shots would cause any problems.  My thought is, if in doubt, take the safest path which in this case would be to keep everything as clean as possible.

Let us know how it shoots afterwards.
 

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Had you checked the size of your chamber throats before you started?  I was planning on fire-lapping a BH i have but the chamber throats are on the verge of being too large to begin with [something that seems to be rather common these days from what i can tell].  So i decided that i'm going to try a 'hybrid' lapping process.  I'm going to hand-lap [VERY carefully, of course] only the barrel with the heavier grits and then fire-lap the whole thing only with the finest [polishing] grit and see how that works [or maybe a few shots with a medium grit but i definitely don't want to increase the size of my cylinder thoats at all which i've heard can happen when fire-lapping].
 

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Beartooth Regular
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Discussion Starter #4
Slamhound,
     I slugged my bore and cylinder a while back. My measurements are perhaps not as precise as some because I used my inexpensive (cheap!) Lyman calipers, but basically I found my chamber throats ran about .4305 and my barrel about .4295 to.430, so I don't expect to have trouble with over sized chamber throats. These measurements were somewhat confirmed by the fact that a lead bullet sized .431 will not pass through any of the chamber throats with light finger pressure, but .430 slugs do. Your point is certainly valid though! Thanks for the heads-up, and I hope your project works out well!.             ID
 

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The Hog Whisperer (Administrator)
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Barrels are MUCH softer than cylinders... it is unlikely that you will increase the diameter of the cylinder throats to any measureable degree by fire-lapping.

There is probably more danger in hand-lapping.

I can assure you, lapping bullets won't open up a cylinder... I've tried!
 

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Actually, as far as the cylinder throats are concerned i am thinking it may have to do with how big they are to start with- i've seen before/after dimensionals posted that seem to indicate that fire-lapping can increase the cylinder throat size on revolvers. I think i'm going to still try what i had planned.  If i be real careful i don't think i'll ruin my barrel.
 

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Regarding opening up cylinder throat dimensions by fire-lapping.  Consider this:  for every six bullets traversing the barrel when lapping a revolver, there is only one bullet passing through each cylinder throat.  Consequently the throats only recieve 1/6th the lapping action as the barrel.  It takes LOTS of bullets to remove a 0.0015" barrel constriction, and I've never seen a gun lapped enough to dimensionally move the cylinder throats!

Just food for thought!

God Bless,

Marshall
 

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That's a good point.  The cylinder throat however is the first place the abrasive-laden bullet has opportunity to contact.  The before/after dimensionals i've seen do indicate that the barrel changes much more than the cylinder throats but in my case i don't want the cylinder throats changing at all so i'm not going to take the chance and still plan on hand-lapping.
 

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The Hog Whisperer (Administrator)
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At the risk of sticking my nose where it doesn't belong....

Let me share with you the story of my first .45 Blackhawk, a 1976 vintage 'convertible.'  .45 Colt revolvers from that era have a reputation for overly large cylinder throats, and this one was no exception.  They all measure .456" or thereabouts, and the barrel groove diameter is .451-.452", don't recall exactly.

With this horrifying mis-match of tolerances, the gun shoots.... just fine, especially with the 300gr. jacketed and 300+ gr cast.  Cast?  No kidding.... if I still had any targets around I'd post pictures as proof.  Perhaps not to the level of accuracy that one would get out of a bench-rest gun, but easily under 2" / 6 shots / 25 yards / open sights with multiple different loads - and I don't brag about my handgun shooting.

Furthermore... when, several years later, I got my Bisley Blackhawk, the opposite problem occurred.  Cylinder throats were .449" or so, groove diameter .451-.452".  Well, I thought, let's kill two birds with one stone.  Fire lap before opening the throats!  About 50 lap rounds later... the cylinder throats were not opened up measureably - I kid you not.  Ruger makes those cylinders HARD.  Worse, the lapping bullets were just wiping the tops of the lands.

Happy ending - got the Bisley fixed and it shoots fine also.  Moral - don't worry about opening the throats by fire-lapping.  Just not gonna happen.  And if they're a little larger than what's considered theoretically perfect -I personally doubt it will make much difference in the grand scheme of things, with good bullets anyway.

Hand-lapping is a known good way to ruin a barrel unless you are one awfully talented gunsmith.  If for some reason I was tempted to do some hand lapping (say, to get the majority of the barrel thread construction out), I'd finish it off with a few rounds of fire-lapping.

If you do have that kind of talent, forgive my rantings (and I've got some work for you also!!!).  

Best of luck no matter which road you choose.
 
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