lapping barrels from .243-7mm / .303-8mm - possible ?

[rosst]

Hi there, a question for the more knowledgable about lapping barrels - while i have read about it ( mostly here ) i have no practical knowledge at all on the subject

in a discussion on my local shooting site a guy stated he had seen 2 rifle barrels lapped out way more than i have ever heard of, a .243 to 7mm amd rechambered to 7mm Muaser and a .303 opened up to 8mm and keeping the rifling sharp during the process . . . has anyone heard or tried anything like this . .. i have my doubts, can't imagine any sharp edges left after the amount of abrasive that must have been used let alone keeping the dimensions stable for the length of the barrel, but as i say i have no knowledge on the subject, time to seek wiser council then . .. opinions appreciated

cheers R

[aspangler]

After seeing what happened to some EXPENCIVE 22 rinfire barrels that had ben lapped by " expert gunsmith" I don't think there would be any rifling left. Those 22 barrels looked like a smooth bore inside after lapping. Barrels had to be replaced. IMHO it would not work. Rebore and re rifle.

[M-Tecs]

He is smoking some good stuff. 100% BS

[Bent Ramrod]

I once bought an old Winchester .25-20 barrel that some amateur had rebored and rechambered to what I thought was .30-30. Somehow he had managed to reproduce the chamber and bore about 0.011" small in every dimension. A full length sized .30-30 shell wouldn't chamber beyond 2/3rds and the groove diameter of the barrel was about 0.299".

I resolved to bring the bore up to 0.308" by lapping. The occasional do-it-yourself gunsmithing article in the pop lit back then warned that a few strokes too many with the lapping rod would take a 25-cal barrel to 6.5mm, so be careful!

I think I gave up after the eighth lap I cast and stroked through the barrel. Each lap was pushed and pulled about 25 times each. I cleaned and measured the bore after every lap session before casting the next lap. The end measurement showed about 0.001" enlargement over the original bore.

So yes, it is possible to enlarge a bore to a larger caliber by lapping. Just depends on how many centuries you want to spend doing the job.

I sent the barrel to Redman's, who did rebores back then, and he turned it into a .38-55. It shoots pretty good. He couldn't enlarge it the next step up to .32 caliber; apparently a certain amount of extra is necessary to ensure clean up of any pits in a barrel. I would guess a .243 could be rebored and rerifled to 7mm (but not .25 caliber), but I doubt a rebore could be done on a .303 to 8mm.

[country gent]

to go from 243- 7mm some one has some strong arms and a lot of patience. Lapping is a long slow process when done properly. removing a couple thousandths takes awhile. Even 308-8mm would be roughly .020 and alot of work. I doubt it was lapped out but also termanology often gets misued. In this case rebored barrels may also have been lapped for finish and consistancy, and the person describing it just settled on lapped. Lapping is also a easy way to "choke" a barrel. 3-5 strokes 0-5" rotating thru rifling then 0-10" rotating thru rifling. 0-15" again rotating thru rifling till end of barrel is reached.

[rosst]

here are his exact words . . . When you lap a rifle bore , you open it evenly , both lands and groves together , you do not smooth off the rifling or even round its corners and edges off, the laps are cast in the bore and changed as size increases , I have seen a BSA 243 barrel lapped out to 7mm then re chambered to 7x57 ,and a 303 Westley Richards lapped out to 8mm/303, all it takes is skill and time and patience.

[M-Tecs]

All of the custon barrel makers that hand lap have you cut one inch min. of the end of the barrel due to the natural belling that happens during lapping. They are only lapping .0001" to .0003" from the bore. .243 to 7mm is 041". That would require 300 or 400 casts of the lap with a total of apx 10,000 strokes.

[Bent Ramrod]

Your correspondent is correct, as far as it goes. Reasonable skill, and infinite time and patience. More than I have, for sure. I'd have to see it to believe it. Ask your correspondent how many strokes he saw the lapper take to get from .243" to .284". And what grade of abrasive he was using.

Mostly, lapping is what you do to get minute tool marks out of barrels and bullet moulds, to eliminate slight constrictions in bores, or to smooth out the tops of rust pits so they tear off less lead from the passing boolit. Changing bore and groove dimensions from one caliber to the next is not in the program. A choke may be lapped in a bore, but it is a matter of a thousandth or less diameter change in the length of the barrel. In the good old days, each barrel maker had his own ideas of how to lap in a choke in a target barrel. This was likely the reason for the reputation for extreme finickyness of some barrels, even those by legendary barrelmakers. They would shoot great in warm weather, poorly in cold; great with certain lubricants, miserably with others, etc., etc., etc.

[bob208]

lapping no way. but there is a process that looks like lapping. it is called freshing out. a lap is cast then a land groove is cut out and a cutter or saw is put in same for the lands. it was done on the old muzzleloaders. I did it one time. never again.

[Ballistics in Scotland]

I don't think it could be done by the process usually considered as lapping. Possibly that much metal could be removed if an unusually coarse abrasive were used until near the final stages. But it would surely round off the rifling. The theory of lapping is that the grit imbeds in the softer metal of the two, remains stationery relative to it, and abrades the harder one. But you would probably need a lap of something harder than lead alloy, to avoid having to change it too often. I also think that the bore would end up inconsistent in diameter.

It might be possible to do it with a copper or brass lap, grooved so that it will work only on the grooves, which is rotated by a rod and a cam or gearing as for rifling. Then (or beforehand) you use another for the lands. But if you are going to do all that, why not just rerifle it?

[DougGuy]

Anybody that remotely thinks they can lap over .040" out of a barrel and have anywhere near something you could call rifling, is dreaming. Re-bore, re-rifle or re-line yes. This is what these processes are for so the bore remains sharp and accurate.

[Ballistics in Scotland]

I suppose you could lap a shotgun barrel, with a rotating lap, to the next bigger size if there is enough metal in the barrel to feel comfortable with. But how often is there? I look occasionally at my single Scott 10ga of the 1890s, which as recoil is governed by weight is far heavier in the barrel than a double. Everybody ought to have an 8ga sometime. But the chances of a reasonable use for it are minimal, and I don't want to make it differ from the proof details stamped on the barrel.

[montana_charlie]

Even if you had the patience to lap .243 to .284, you would still have the same twist rate in the rifling.
Would the 'standard' .243 twist work for a 7 mm?

[Ballistics in Scotland]

Were that all, it would work well for light bullets, and not for the heavy ones that have a long tradition of performing well in the 7x57. But that isn't all.

[M-Tecs]

I have only hand lapped a couple of barrels but I have watched John Krieger lap barrels and I have spoken in length with both John and Boots Obermeyer on barrel making and lapping so I am not totally unfamiliar with lapping.

Once the lap is cast and the abrasive is applied it cuts on all surfaces. This includes the sides of the lands. The lands would get to be.041" narrower. With that much lapping you will have significant belling on the ends of the barrel and rounding of the top of the lands if any are still remaining.

Cut rifled barrels for jacketed bullets are normally 30/70 land/groove ratio. Six and eight groove barrels the lands would washout and the barrel ID may take a Polygonal rifling form.

Lapping .041" out of a barrel and having a useable product is not possible regardless of the skill and patience of the person doing the lapping.

[rosst]

thanks guys, your replies are very much appreciated . . . you have i think confirmed my first thoughts that something is wrong when i read the post . . and thats important to me. i dont mind a story growing a little in the retelling, thats normal . . but handing out stupid advice annoys the hell out of me . .. thanks again . . . R

[Mk42gunner]

Not even getting into the lapping process; I seriously doubt that a 7X57 chamber reamer will clean up a .243 Winchester chamber.

Robert

[GabbyM]

Me thinks you just heard some butchering of the English language from a person unfamiliar with various shop practices. There are shops that re-bore barrels then recut new rifling.

The most abused term around this site is smelting.

[Cap'n Morgan]

Iv'e seen Schultz & Larsen barrels being lapped. It is still done in an ancient vertical lapping machine - three barrels at a time if I remember correctly.
The stroke motion is accomplished by the way of a crankshaft. Laps are cast from pure tin and grinding compound is added through the muzzle during the process. For best precision the barrels should taper slightly towards the muzzle - about .0004 or so.

[pietro]

in a discussion on my local shooting site a guy stated he had seen 2 rifle barrels lapped out way more than i have ever heard of, a .243 to 7mm amd rechambered to 7mm Muaser and a .303 opened up to 8mm and keeping the rifling sharp during the process . . .

IMO, that's a classic keyboard commando statement by somebody that doesn't have a clue.


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