Fire lapping techniques

[gcf]

Guys, the following is old hat to some of you, I'm quite sure.

That said - for the benefit of others - attached below is a picture of the Beartooth technical guide, that among other things, provides clear cut directions for effective fire lapping. Veral of LBT also wrote a similar book, that may still be available. Both of these guys know what they are doing - with regards to shooting cast bullets. If at all possible, I'd recommend laying hands on either - before diving in to the fire lapping process.

The recommendation outlined in the BT tech manual, is 320 grit compound (I believe the BT compound is 320 grit Clover re-labeled) embedded in SOFT lead bullets, and smeared into the bullet lube groove, & then fired at low (600fps) velocity. Followed up with a compound bore polishing session. A lot of (non-random) thought went into this process. The idea being to remove constrictions, irregularities, and provide a fine polished finish - without expanding the bore ID to the point that it will function with a larger caliber.

Not to say that a bit of thoughtful process variation can't be employed though. Fermin's interesting process outlined, is a variation that adheres to the original. I myself,  thoroughly clean the bore, & lube with CLP after every cylinder full of lapping loads - and am convinced that it produces a more nicely polished bore, & generally enhances the process.

Now, hopefully without stepping on any toes, and understanding that everyone's mileage varies, I'll say that JUDICIOUSLY following the basics ( 320 grit compound embedded in soft lead bullets, and / or smeared into the bullet lube groove, then fired at low velocity) while checking progress frequently, is the safest route to follow - for anyone who values their firearms. As someone smarter than me once opined, it's a heck of a lot easier to remove metal from a bore, than it is to replace it.

Again, JMHO...

[niederlander]

That's what I've been doing. Up to 131 rounds so far. It's better, but we're still not there. I've never seen a bore constriction like this, but understand it's not all that uncommon with Ruger big bores. I'll have to say, constriction aside, the rest of the bore is now the smoothest I've ever seen!

[bigbore5]

131 rounds? I haven't used more than 35 on a stainless Service Six that had throat constriction and chatter marks the whole length. I only lapp until the groups close up and leading is gone

[niederlander]

Seems odd to me, too, but it is what it is. I started with LBT's compound, then to 400 grit Clover because it's what I could get at the time, and now have 320 grit Clover. Hopefully, that will do the trick.

[gcf]

Sept 6, 2024 13:18:46 GMT -5 niederlander said:

Seems odd to me, too, but it is what it is. I started with LBT's compound, then to 400 grit Clover because it's what I could get at the time, and now have 320 grit Clover. Hopefully, that will do the trick.

Might suggest you get your hands on a set of pin gauges. Fairly cheap at Amazon. They will tell you how much of a choke point (within 0.0005") you actually have, and help you track your progress.
Handy for checking cylinder throats, as well...

[niederlander]

I've used them. If you're familiar with the Brownells 11 degree forcing cone cutter, they have a gauge to see if it's the proper depth. It slides in perfectly at the muzzle. It won't go in the breech end. It's getting better, but I need to shoot some more through it.

[niederlander]

Also, looking on the bright side, I'm getting pretty good at making up these fire lapping rounds. Takes me about twenty minutes to make eighteen of them.

[niederlander]

I AM beginning to wonder if I'm doing something wrong. I'm up to 161 rounds fire lapping, and while the constriction is better, it's still there. Wondering if the barrel was defective to start with.

[gcf]

Sept 9, 2024 16:24:55 GMT -5 niederlander said:

I AM beginning to wonder if I'm doing something wrong. I'm up to 161 rounds fire lapping, and while the constriction is better, it's still there. Wondering if the barrel was defective to start with.

Defective? Maybe, but more likely caused by a thread advanced timing issue, and over torqueing of the barrel (by who ever installed it), to bring the front sight up to top dead center.

Our own David Bradshaw describes this in detail, in the last post of the following (Taylor Throating) thread. It's good info, and worth a look: 

singleactions.proboards.com/thread/2587/taylor?page=3

It's unclear to me, how much of a constriction, or bore irregularity we can reasonably expect to resolve through the fire lapping process. Thinking 0.002" - 0.004"... MAYBE??

Got to say, 161 FL rounds seems somewhat excessive. Especially with a non-stainless barrel. Sorry you're having the problem. I'm certainly no gunsmith, but assuming that the bore rifling is still OK, possible solutions might be setting the barrel back, or Taylor throating. Either performed by a truly competent smith. Guessing your pistol is worth it...

[2 Dogs]

Sept 9, 2024 16:24:55 GMT -5 niederlander said:

I AM beginning to wonder if I'm doing something wrong. I'm up to 161 rounds fire lapping, and while the constriction is better, it's still there. Wondering if the barrel was defective to start with.

Sir, why don’t you give me a call. Something’s definitely not right.
Perhaps I can help you resolve it.
Fermin
361-960-3697

[niederlander]

I think I'm actually getting pretty close. I'll try a few more rounds and call you if that doesn't work. If all else fails, I guess I can always re-barrel again!

[niederlander]

Taylor throating may end up being the answer. Is that done something like the Brownells forcing cone cutter works?  Is there a cutter available for that?

[1lnbrdg]

38_148grWc (813.83 KB)
I'm fire lapping a Taurus 608 revolver, stainless steel barrel, up to 96 rounds lapping shots currently. TKCustom worked the cylinder over while converting it to moon clips. I shot this group yesterday after the last 24 lapping rounds, 35 yards. It was getting about 2" of leading streaks ahead of the forcing cone when I started, now down to about 1" streaks after the last 32 rounds. It started with shooting 6" pattern at 35yds so there's been a lot of improvement. 

Also did 24 rounds in my .44 after getting the cylinder back from Fermin, a ways to go on it yet, still a lot of leading and no lube star. Still shooting a pattern instead of a group. 

[1lnbrdg]

Oct 29, 2024 9:15:30 GMT -5 1lnbrdg said:

I'm fire lapping a Taurus 608 revolver, stainless steel barrel, up to 96 rounds lapping shots currently. TKCustom worked the cylinder over while converting it to moon clips. I shot this group yesterday after the last 24 lapping rounds, 35 yards. It was getting about 2" of leading streaks ahead of the forcing cone when I started, now down to about 1" streaks after the last 32 rounds. It started with shooting 6" pattern at 35yds so there's been a lot of improvement. 

Also did 24 rounds in my .44 after getting the cylinder back from Fermin, a ways to go on it yet, still a lot of leading and no lube star. Still shooting a pattern instead of a group. 

[bigbore5]

I lap and finish parts for a living. I'm really curious about what kind and grit of compound that is being used. I start course as necessary and progress to finer, usually a 3F followed by 1000 does the trick for fire lapping. I only use just enough of the 3F compound to cut out any thread choke and tool chatter, then the 1000 polished very smooth.

The worst barrel I have ever done successfully was on a Service Six. Thread choke and severe chatter marks all the way up the barrel. I started with 18rds Clover 120, then 12 Clover 360. 18rds 3F then 18 of 1000. Load produced around 600fps. Bore came out very shiny and it's now one of my most accurate guns despite the tiny fixed sights