Do I want to lap my bore?

[kappy]

The question says... some of it. I'm specifically looking at my Mosin M39.

1. Why do I want to do this?

2. Can I do this myself?

3. What would I need?

4. What am I looking at, cost-wise?

5. Any downside?

[405]

The question says... some of it. I'm specifically looking at my Mosin M39.

1. Why do I want to do this?



Well that IS the question. Who knows the answer? :)
And who knows if you can do it? :)

As far as the others... no it's not hard or expensive to do. Bores can be fire lapped or hand lapped.

[longbow]

I'm with 405 ~ why do you want to do this?

If you don't have a good reason then why do it?

Is the bore rough ~ pitted or machine marks?

Have you slugged it to see if there are tight spots?

I have only lapped one bore for my 1894 Marlin which suffered from apparently well known tight spots under dovetails (I didn't know about them). The gun leaded badly and accuracy was pretty mediocre.

Once I read about the tight spots I slugged the barrel and found... tight spots. They weren't obvious with a tight patch on a jag but were when slugged.

In my case I used a .440" ball as a lapping slug after drilling a hole in it and driving it into the bore then attaching to a long 5/16" steel rod. I put some valve lapping compound on the ball then proceeded to work on the tight spots. I used a bore guide to keep the rod away from the bore.

More commonly, a lapping slug is cast in the bore. I just figured this was easier and it worked for me.

After the tight spots were removed, I made a few full length strokes and was done.

No more leading and much improved accuracy.

It does not take a lot of equipment, time or money if you do it yourself.

Whether you need to do it and have the skill to do it are the questions. If you don't need to do it, don't. If you don't want to take a chance on ruining the barrel, don't do it.

The upside is that you can improve accuracy and reduce leading if the gun is not shooting well due to bore condition. The downside is you can ruin a barrel.

If the bore is slightly rough, then a little firelapping may smooth it up and improve things and is unlikely to cause any damage unless seriously overdone.

Just my thoughts.

Longbow

[kappy]

Thanks for the responses.

I guess what I'm really saying is... "I don't understand what it does, but have heard it makes rifles more accurate." I was curious as to whether it makes ALL rifles more accurate, like a good trigger job, or if it is only necessary in certain cases.

The rifle doesn't seem to have any pitting or machining marks. Those Finns sure as hell seem to know what they were doing.

[Char-Gar]

No you don't want to lap that bore. Those rifles have been in somebody's military service for generations and through wars. The barrels have had plenty of use as it.

[gnoahhh]

The only time I would think of lapping an old foresaken mosin barrel would be to gain the experience, which has merit. Lapping something like that isn't a panacea when you consider all the ills it probably has due to what Char-Gar said. Countless rounds of corrosive ammo fired by semi-literate soldiers with only a passing knowledge of proper bore maintenance has left very few of those clunkers with anything resembling a crisp pristine bore. (If you don't believe me, look at a few seemingly pretty Russian bores- or any other decades old milsurp- with a borescope.) Better to expend the money, time, and energy on more deserving rifles, IMO, and use the old girls as you found them and work around their foibles.

[nanuk]

I bought an older 30cal remington barrel to practice on... $40 is not too much to waste.

[nicholst55]

I suspect that almost as many barrels have been ruined by unnecessary lapping as by rust.

[Mk42gunner]

Go to the lasc website and read Mike Bellm's thoughts on lapping a barrel.

Back in the pre-internet days, I lapped the barrel of my Uncle's flintlock that he had allowed to rust. It had some rather severe rust and pitting for about six inches about eight inches from the muzzle. The rile shot okay after that, but not nearly as well as it did when new.

It was a shame really, it was a good .50 caliber Sharon barrel to start with.

Robert

[nicholst55]

Go to the lasc website and read Mike Bellm's thoughts on lapping a barrel.

Robert

About Fire Lapping Barrels (http://www.lasc.us/bellmFirelappingBarrels.htm), by Mike Bellm

To summarize, Mike recommends against fire lapping, unless you're going to rechamber.

[kappy]

No you don't want to lap that bore. Those rifles have been in somebody's military service for generations and through wars. The barrels have had plenty of use as it.

Supposedly (and it's a BIG supposedly), the rifle was in "unissued" condition. In its defense, the bore was pristine when I purchased it. Several local guys said it was the prettiest military bore they'd ever seen.

Regardless, I take everyone's point here. I shall do without the lapping.

Besides, today I did (for me) very well at the range.

[MtGun44]

Lapping is generally a process for correcting a problem. Hand lapping can be done to just
polish the bore, removing a negligable amount of metal and smoothing the bore. This can
make the gun easier to clean and MIGHT improve accuracy a touch.

Fire lapping is a much less controlled process, but - if there is a problem, it may be able
to improve some specific problems.

Bill

[trench]

I know a guy who ruined 2 308 rifle barrels with the firelapping, and he is nobody's fool.

[smokemjoe]

Dont use valve grinding compoud inbedded in a cast bullet, good way to ruin a barrel with in 10 rds.

[303Guy]

The trick with fire-lapping is to force the compound into the corners and grooves. I think the best way is with an abrasive paste cookie under an oversize paper patch boolit with a wad under it and don't use squib loads - use starting loads. Only two or three are needed and the job's done. Have a look under smokeless paper patching for my thread on it. I'm talking restoring a rust ruined bore to shootability.

http://castboolits.gunloads.com/showthread.php?t=135326

[johnly]

I had a factory M700 338 winchester magmum stainless barrel that copper fouled badly. I fire lapped it with cast bullets starting with a couple of rounds of 220 grit. Cleaned it, 3 rounds of 320 grit, cleaned, then 6 rounds of 600 grit. No more fouling, first shot out of a clean barrel is in the group and the next two will form a group of less than an inch with 76.0 gr of RL-19 pushing a 200 grain Nosler Accubond. It's now an accurate elk killing machine if I do my part.

If your rifle doesn't foul, there's no need to lap it, but if it fouls or just doesn't shoot well there is not much to lose.

John