Barrel lapping

[Denver]

Howdy All;

I'd like to hear from anyone who has lapped their rifle or pistol barrel(s) ala LBT. Does it help? Can it be done using lapping compunds other than those sold by LBT?
I read somewhere that using silicon carbide abrasives is not the way to go, but to use aluminum oxide instead.
I'd appreciate any info you have to pass on,

Thanks, Ron

[Bass Ackward]

Howdy All;

I'd like to hear from anyone who has lapped their rifle or pistol barrel(s) ala LBT. Does it help? Can it be done using lapping compunds other than those sold by LBT?
I read somewhere that using silicon carbide abrasives is not the way to go, but to use aluminum oxide instead.
I'd appreciate any info you have to pass on,

Thanks, Ron

Ron,

I never recommend that some one try to lap by hand. Shilean will only let their most experienced guys do it and claim that even some of their work has to be down graded after because it doesn't meet tollerances. Shilean has one of the best reputations for "bore tollerances" as far as diameter goes. But also the worst reputation concerning consistency of twist rate. This is what you CAN'T control by hand. What good does it do to solve one problem and create another?

If you are talking about fire lapping, then I am much more familar with that because I have fire lapped maybe 50 to 60 barrels. The lap compound sold by LBT works fine. The keys to it are regardless of who's kit you buy.

1. Always use lead instead of copper.
2. Stop and slug often. Shoot slow and low pressure.
3. Quit BEFORE you get to where you want to be.

The problem with copper is rapid erosion that is hard to monitor without ruining your throat and cutting valuable height off your rifling.

The problem with two is that after you rough up the bore, it is hard to feel if you have solved your problem which is the only reason to fire lap in the first place.

The problem with three is that after you rough up the bore, you have to start all over with barrel break -in. Shoot clean, shoot, clean. And the jacketed will close the pores of the metal as it wears away the rough spots. You can lose another .0005 during this time. So if you lapped to where you want to be, you are screwed.

Anything more that you might want to know, PM me and I will try to answer.

[longhorn]

I firelapped several Ruger revolvers that seemed to suffer from a bore constriction at the frame (this is based on "feel" with tight patches and cleaning rod, not accurate measurement!) and rough forcing cones. Leading decreased noticeably in all (with too-hard commercial bullets and low pressure loads). No improvement in accuracy, except for continuing consistent groups in the absence of the lead. Go very slowly.

[Ballistics in Scotland]

The best tool of all for measuring a bore is a set of pin gauges, which is cheaper and more versatile than a proper bore micrometer. A set of .25 to .5 by thousandths can cost under $40 on eBay. They will measure only a land diameter, but that will detect a constriction, and you don't have the problem of needing different micrometers to measure barrels with different numbers of grooves.

I think a bore tapered the wrong way is much worse than small departures from either absolute correctness of diameter or consistency of twist. After all, people get good hunting accuracy with gain twist barrels, and there are dark stories of people doing the same after accidentally installing one back to front. Within the limits good makers can do this in a barrel intended to be constant twist, I don't think it's anywhere near the worst thing that can be wrong.

A technique which I've never heard of anyone trying, and mention as a subject for experimentation, is fire-lapping with sections of anodised aluminium rod. The coating simply is aluminium oxide, although so thin that my small slitting saw has survived a lot of cuts in 8mm. rod without bluntening. With its hardness and low inertia, I don't believe it would upset at all.

.375 barrels have their own size all ready, and 6/16 should be right for .303. I don't know about the same in a .30 calibre, but testing with increasing loads and a 1/4in. long slug of the stuff should tell you if pressure signs are high.

[KCSO]

For bore lapping I cast a lead lap in the bore and use commercial lapping compound mixed in bear oil (its what I have). I ALWAYS use a steel rod and a bore guide and start my lapping at the breech end. I try to do 100 strokes at the breech third, 50 in the middle and 25 at the front. Lap 10 each and then clean,recharge, lap the full bore twice and then repeat till you get the bore you want. What I try for is the bore described by Ned Roberts for the paper patch bullet. It should take some resistance for the bullet to go down the first 2 to 3 inches at the muzzle and then should slide free to the breech. This takes a LOT of feel and some experience. I don't usually lap a pistol barrel unless it is pretty rough, as if the forcing cone is smooth a box or two of jacketed loads and a good cleaning between boxes will usually do the job. I had lapped a 32 barrel for a custom Ohio target rifle this week and we tried it out today. With peep iron sights it put 5 in under a dime at 25 yards. The best part was loading and cleaning as the smooth bore loads a lot easier than the rough tube was doing. I have never been impressed with fire lapping so I can't comment there.

[Molly]

This is a quick and easy bore clean / polish technique that has worked very well for me, and for a few others too. It was written up in the CBA's 'Fouling Shot' magazine, and others have since reported excellent results with the technique. But if you aren't a CBA member, here's the short version:

The first trial occurred because I bought one of those Yugoslav 8x57 rifles that were being sold as new, unfired military surplus for a pittance. Well, to judge from the cosmoline and condition, I believe that’s exactly what it was. But the bore, though obviously not worn or pitted, wasn’t very smooth either. I could perhaps illustrate it as looking more like the surface had been finished with 400 grit wet-or-dry paper. It may have been new, but it sure wasn’t polished either.

I’d done a lot of reading about fire-lapping, but for the life of me, I simply couldn’t see the logic of pushing abrasives as coarse as what I’d seen recommended down a good bore. And the reports of throat enlargement lent some credence to my reservations.

No doubt I could have used the rifle as is correction - as WAS - for the rest of my life with jacketed bullets, but I’m a cast bullet enthusiast, and have been for decades. I wanted to use it with cast bullets, and a rough bore isn’t commonly considered optimal for them. So I gave the concept of fire-lapping some serious thought. It seemed reasonable that the finer the abrasive, the less likely I was to encounter serious enlargement problems, and I was willing to accept a longer process for this assurance.

I’m a long-time fan of JB Compound too, and it seemed like a reasonable choice for the trial. I was thinking of using cast bullet, mostly because I could hand-lube their grease grooves with JB, but then again, I had all those pulled slugs from milsurp ammo - I decided to use them instead, and simply use a bore mop between shots to introduce the polish.

I pulled down some ammo, weighed the charges, and reduced it to provide a safety factor. It was well I did, because I did obtain some substantial pressures on the first few shots. But when I ran a patch down the bore to check progress, I was astonished to see that it gleamed like someone had coated it with mercury! But measurements with lead slugs before and after didn’t show any bore enlargement. I suspect that there was some - polishing being a metal removal process - but it was apparently darned little! I wrote it up for the Cast bullet Association, and they’ve since published reports that others have tried it with excellent results. One guy had about ruined the bore of his match rifle, but restored it’s finish and accuracy with this process.
Equipment:
Cleaning rod
Cleaning solvent
Bore mop
Patches
Cleaning solvent
JB compound
Specially loaded ammo (See special notes below)

Procedure:
Take everything to the range, as proceed as follows:
1. Remove the bolt from the rifle.
2. Coat the bore mop with JB compound.
3. Swab the bore to leave a LIGHT coating of JB compound throughout.
4. Replace the bolt.
5. Load with special ammo and fire.
6. Repeat for about half a dozen rounds, swabbing with JB compound between each shot.
7. Clean the bore to check on progress / improvement.
8. Repeat as necessary.

Special notes:
1. Used with some common sense, this process works beautifully and quickly: It will leave a bore mirror bright if everything is done correctly, but will result in little or no enlargement. But it CAN be hazardous if the following warnings are not carefully observed.
2. You MUST NOT use factory ammo. This procedure will elevate pressures, and if factory ammo is used, the results could be very dangerous.
The reason is at least two-fold: the JB compound will increase the friction between the barrel and the bullet quite a bit, requiring much higher pressures to push the bullet down the bore.
Also, the paste will reduce the clearance at the case neck. If the chamber neck area is too heavily coated with JB compound, the case won't be able to expand and release the bullet properly. This can also raise pressures quite a bit.

My first trials used military ammo from which I’d removed a fourth of the powder charge. Even so, I experienced very high pressures for about three shots, though I didn’t notice the pressure signs (badly flattened primers) until I’d finished. After that, the cases all had perfectly normal pressure signs. I found that removing at least 1/3 of the powder charge from a factory round seems to give very satisfactory results in my experience, but I will probably use a 50% powder charge for extra safety from now on. And a heavy coating of JB compound won't work any better or faster than a really thin coating, so use some common sense.
3. I used jacketed bullets, and got fine results. A new rifle (nut with an obviously rough bore) gleamed like chrome after about five or six shots. A pitted bore will undoubtedly require more shots. And I strongly suspect that if you want to use cast bullets, you may get somewhat slower but still safer results.
4. Cast bullets may have another advantage too: The 6.5 Carcano's generally have a BORE of about 0.264, and a GROOVE dia of about .270 to .272 in my experience. Handloads with .264 jacketed bullets require base expansion to really grip the rifling and spin the bullet. With light loads, all you may be able to do is polish the tops of the lands. Cast bullets will expand much more easily, and fill the bore grooves for more uniform polishing. Or if you use milsurp jacketed bullets, they are larger than .264, and may work better too.
5. As an extra safety margin, I recommend firing the rifle from the bench (not the shoulder) with the bolt and breech covered with a couple of sandbags. I think this should be adequate to provide greatly improved safety even is everything else goes wrong.

Sorry about all the potential hazard warnings, but they ARE just potential - not necessarily probable - hazards. I think the process is plenty safe if used by someone with ordinary experience and caution, or I wouldn't mention it to you. But I have no way of knowing the level of caution you consider appropriate. So this information is for your information and consideration only; it is not a recommendation to be implemented blindly. Use at your own judgment and risk.

[Nelsdou44]

Ron,

I've used the 320 grit silicon carbide in a grease paste and I think it works just fine.

A couple of things that I can add to what's been said: Using a soft lead bullet with the grit embedded into it helps to cut the tightest restrictions of the barrel first as it sizes down to that smallest diameter. So as you progress round after round you will get a slight taper towards the muzzle. The other thing is in using squib loads (keeping velocity low) is if you get a cast bullet stuck you can tap it out fairly easily with a brass rod.

I had an old mauser barrel that was a tomato stake candidate for accuracy and following the instructions much like BA has above brought it back to near MOA accuracy for jacketed bullets. Truely amazed how a tight patch slides down that old barrel with nice even pressure on the rod. Still searching for that cast bullet/load..........

Just my opinion, but I think damage to the throat can be minimized by not using excessive loads and using only the necessary amount of grit needed to get it embedded into the slug and no more.

Nels

[uscra112]

My vote goes with KCSO. The only careful analysis of firelapping ever done by the American Rifleman tech staff, (back when they actually HAD one) showed that the main result was to drastically lengthen the leede and round off the leading edges of the rifling. Very little if any effect was seen at the far end of the bore. They actually MEASURED the results with star gages and studied the surfaces with borescopes.

This makes complete sense. Lapping, when it's done in manufacturing, (I'm not talking about barrels) is always done at very low surface speeds. Certainly not 800 feet per second. Maybe 800 feet per MINUTE. This is to keep heat buildup from distorting the work. In this case I think it's also a heat issue, but a more pernicious one. A lead boolit has to heat up as it goes down the barrel, and with increased temp the lead will soften and let the abrasive grains sink into the surface, making it impossible for them to do any cutting. Hence lots of effect at the breech end of the barrel, little or none at the muzzle.

Another theory would have it that the boolit surface (and hence the grit) is held off the steel by the lube. This also makes sense to me, having plowed through tomes such as "Theory of Hydrodynamic Lubrication" in my mis-spent youth. Said theory can be said to hold that lube films work better and better the faster you go, (until you get to a point where heating causes the lubricant molecules to come apart).

Molly's technique will probably get around this, and may be OK for polishing. For actually removing significant metal, I dunno. T'would seem to be very uncertain and uncontrollable to me.

The only time I'd firelap anything would be revolver forcing cones - if I were a handcannon junkie which I'm not.

I haven't ever seen any actual measurement data coming from these firelapping kit sellers, and if they had any I doubt they'd want it published.

The true geniuses of lead-boolit barrel making - the Popes and Shoyens and Schalks and Brockways, would have looked at you in slack-jawed amazement if you mentioned firelapping in their presence, and if you happened to have one of their barrels they would have taken away from you and slapped your wrist with a shingling hatchet.

There is no substitute for proper rod lapping.

As far as that Shilen story is concerned, I'd have to see the video tape before I believed any of it.

[StrawHat]

Help me out here guys.

How does lapping a barrel affect the rifling twist????

No amount of lapping (shy of making it a smoothbore) will alter the twist if I understand what you mean by lapping and twist.

I do understand you can introduce a tapering or choke effect by lapping.

Maybe I am confusing terminology?

Thanks.

[Molly]

> How does lapping a barrel affect the rifling twist????
>No amount of lapping (shy of making it a smoothbore) will alter the twist if I >understand what you mean by lapping and twist.

That's correct.

> I do understand you can introduce a tapering or choke effect by lapping.

That's also correct. Many old-time rifle makers would lap a barrel progressively. That is, the entire bore would be lapped for uniformity, until passing the lap through the barrel indicated no detectable tight or loose spots. Then a new lap would be cast, and the bore would be lapped again from the breech to within a few inches of the muzzle. This gave a choke bore, or a choked bore. The depth of the enlargement, or the amount of the choke effect was entirely up to the whim of the gunsmith.

If a new lap was again made and used to within a few inches of the previous end point, and this was continued to progresively enlarge the bore from the muzzle to the breech, the result was called a taper bore.

Though highly labor intensive, these procedures were - sort of - practical when there were no mass produced jacketed bullets. The bores so produced were not standardized in any way: Diameters depended entirely on the whim and experience of the lapping gunsmith. However, this was not a real problem when the bullet mold that accompanied each new rifle could be made to produce a bullet that would fit the bore properly. However, with modern standardized bore and projectile diameters, hand lapping has pretty much fallen into disuse. It can still be useful to salvage a verly lightly rusted bore, or to remove a tight spot, but that's about it.

[Moose]

Best approach lapping and esp. 'fire-lapping' with all due caution. Try it out on something like a rough shotgun barrel. The idea of firing grit-laden ammo down one of my pieces - well, you kick your nephew in the b**t for picking up that dropped round and sticking it back in the chamber, no? Some experiences with the lapping procedure show that A) you can remove a lot more metal than you want, and B),do it quicker than you thought. Lapping works, but it is not as simple an operation as some folks would have you believe.. If you want to try it out, get some auto body rubbing compound in "aggressive" grade, some tough, tight fitting patches, a proper diamond-cut jag and a good rod, and do the bore paste polish thing. Could be just the thing, and no melting/pouring/stamping and cursing. You will have to strip the piece, but you would have to do that to lap it. Good luck and don't expect los milagros.

[Bass Ackward]

Help me out here guys.

How does lapping a barrel affect the rifling twist????

No amount of lapping (shy of making it a smoothbore) will alter the twist if I understand what you mean by lapping and twist.

I do understand you can introduce a tapering or choke effect by lapping.

Maybe I am confusing terminology?

Thanks.

Straw,

Twist rate is determined by a certain width band rotated at a certain angle to complete one rotation. As you change the rotation rate, you change the twist rate. If pulling a button and you hit a harder piece of steel, the machine can lug and elongate or change the angle slightly. And no amount of cast shooing will ever correct it. Well not in your life time on today's steels.

So if you are hand lapping, you will be inadvertently working the upright sides of the twist rate which is much more important to cast than any short diameter or roughness problem because pressure can keep the seal. On a change in twist rate issue, pressure up the body of the bullet has less effect than on the base to seal. You can hand lap until the cows come home and never fix this. The negative to hand lapping? You can actually cause this problem somewhere else as you stop and start again. Or you can open up your bore trying to corect the stoppage and actually make it worse. You are watching horizontal progress and not mindful of the vertical damage.

This is where the only thing that can correct this does. The fired bullet. What actually happens is the the rifling width is actually diminished to allow smooth and continuous travel. Rotational forces provide the perfect rotater so each land eventually pulls evenly. Otherwise maybe one carries the load and this increases how hard your bullets need to be to survive it. (Why some guys may need hard lead for greater than 2000 fps. even though a patch glides like nobodies business.) The off side of the land will be corrected by friction of the next land as the bullet tries to maintain distance spacing.

Many times people feel this with a patch or handlap and ASSUME it is a constriction. THEY THINK in one dimension only! (You can't see it, how would you know) And remember, if getting the round surfaces (groove and bore) can have problems that are easy to make, then the vertical sides of the rifling can have just as many if not more. Especially, at the junction of the land and groove. Now if this defect is too long for your bullet bearing length, (even after correction) then you will need a heavier bullet to reach across this imperfection and maintain seal. Or you will need a harder bullet.

Happens with copper too which is why you get heavy coppering in spots. The assumption by a one dimensional thinker? Rough spot. Most coppering is actually caused in a barrel by gas cutting more than friction. And some guy may "think" he broke in his barrel and that he just wasted his time doing it. He actually hasn't gone far enough.

And this is why I prefer to let my bullets do the lapping even if I must sacrifice some dimensional enlargement or throat wear to get everything to pull evenly. I always test the top end of my barrels with lead before and after and I have always seen an increase in velocity and accuracy on the top end. Especially with softer lead.

Misunderstanding of this problem has caused three commonalities to develop for cast. 1. Best accuracy occurs around 1000 to 1600 fps. 2. Heavy bullets per caliber are more accurate. And 3. harder bullets are more accurate than soft. All are true if this problem exists.

[mhb]

should only be done by the barrel maker, and is practically universal in match-grade barrels made by the various methods in current use. The reason is that lapping corrects inadvertent minor dimensional variations in the bore, and improves surface finish while removing any remaining burrs or sharp edges on the lands - but lapping unavoidably introduces dimensional variations at both ends of the blank, where the lap is charged and passes in-and-out of the bore. To correct this, the barrel maker customarily removes the fixtures (end segments specifically dimensioned to locate the barrel during drilling, reaming and rifling) before the barrel blank is shipped to the buyer, or fitted and chambered, thus eliminating the suspect end portions of the barrel. If the barrel is lapped by any means in the completely chambered and fitted state, the unavoidable damage done at the breech end, particularly to the chamber mouth and throat, cannot be corrected, and, unless the muzzle is shortened, the muzzle (most important in accuracy) may also be adversely affected.
Lapping cannot correct excursions in rifling pitch, which are often encountered in button rifled barrels; neither can a gain-twist barrel be lapped. And, consider, the lap itself, of whatever made, must be soft enough to be permanently deformed by passage through the rifled bore, so cannot be counted on to make any sort of uniform correction to rifling which varies in pitch from place-to-place in the bore. Some improvement of surface finish of the lands, and removal of burrs, if present, can be achieved by a bore-riding lap, of bore diameter only, which cannot affect the finish in the grooves.
And, truly, lapping should only be done by the traditional method of casting a soft-lead lap several inches long in the barrel blank, charging it with abrasive of the correct grit, and casting a new lap for subsequent, finer grits until the desired surface finish is achieved. Tight patches cannot do the job, and fire-lapping is so rough-and-ready a method I recommend against it unless the barrel is known to be inaccurate due to problems related to bore surface finish, cannot be corrected by any other method, and you can't afford to replace it.
One of my friends, for whom I built a .223 match rifle, inadvertently fire-lapped the throat right out of his rifle by failing to remove all the J-B bore cleaner (which is NOT an aggressive abrasive) from the chamber after cleaning the barrel - he lapped the throat right out of there in about 400 rounds - I was able to save the barrel, a long, heavy match type, by setting it back and re-chambering it.
Best leave lapping to the barrel maker, except in the direst of circumstances, and then be prepared for undesired results.
mhb - Mike

[uscra112]

To an extent I would agree with Bass Ackward about the effect on twist, IF one is using a proper lap that's several inches long.

In the machine tool business where I used to work, we would occasionally lap new-made leadcrews (Acme thread) with what was essentially a very long nut, and the sole purpose was to make the screw pitch as uniform as possible. The position of the nut on the screw must be thought of as the AVERAGE of all the points of contact. As you went up and down the screw, the lap would bear heavier on the "high spots" and even them all out to match the "average". (It also tends to even out the "nut".) The basic principle goes all the way back to the beginning of the Industrial Revolution, when people were trying to make an accurate leadscrew for a lathe.

A rifle barrel is no different, except it's inside out. It's essentially a very long nut, with a very long thread pitch.

BTW the thread-gauger's term for what would happen if the carbide button were balked in its' travel down the hole is "drunkenness". No foolin!

[Bass Ackward]

Nicely written Mike. And you guys are absolutely correct. Especially when we talk copper.

The basic thrust of my hand lapping and fire lapping philosophy was garnered from a conversation with Ol'man Shilen back in the 70s. Let me make sure this is clear, he was not advocating fire lapping as is thought of today. In fact we didn't know what that was back then. Fire lapping was shooting jacketed bullets down a bore. He was talking strictly copper.

Our discussion was at an NRA convention and we were specifically asking why it took so long to get air gauged barrels from them. His comment was 50 shots and you have the same barrel or maybe better. He said that any lap, large enough to be well fitting to do the job in one direction, would be too hard to pull through by hand. Firing was the only way to get true uniform dimensions. He also recommended that you remove 2" from both ends of the barrel especially if you were not going to indicate up and intending to have the reamer follow the bore. Common chambering procedure back then.

But we have new lapping materials available today that not only remove basically no metal, but are used by long range competitors every so often or so to "extend" barrel life. Read the man's info and judge for yourself as he claims that worn barrels can be recovered using this method.

http://www.jarheadtop.com/article_finalfinish.html

I will admit, you can ruin anything if you are not mindful of a goal.

Consider a good barrel for cast is different from copper. Many guys like to choke or certainly go .001 - .002 over their bore diameters. Cast benchresters have found that 1/2 degree throat tapers work best for cast. And rifling that pulls copper will destroy lead. So to me mind you, (to each his own) fire lapping that results in a slight taper into a bore with the leade wearing to a much more cast friendly angle, into a bore that has been maybe funneled slightly at the throat to smooth the transition and the bore opened slightly to clean up imperfections that results in slightly rounding rough or sharp edges without affecting rifling height, sounds amazingly like the definition of a top notch, high velocity cast barrel to me.

[mhb]

Having been a machinist, toolmaker and barrel maker, I can't agree that the external lapping of a lead screw and the internal lapping of a rifle barrel are at all comparable - I've done both; admittedly not so many lead screws as rifle barrels (by a factor of hundreds). Lead screw threads are mechanically generated, and vary little in pitch, while button-rifled barrels, particularly, depend on the pitch geometry of the button itself to generate the rifling pitch down the bore (that is, the button rotates itself to generate the pitch), and can show quite large incident variations in pitch from point-to-point in the bore. This is because the forces on the button are truly enormous, and softer or harder spots in the barrel material, variations in the lubrication of the bore or dimensional variations in its diameter cause the button to skip or stutter in its passage through the bore. It is true that some button-rifled barrel makers attempt to minimize the variations by 'leading' the button - driving it on a fixed pitch mechanically produced, but the forces on the button itself render this, at best, a moderating influence - the torque forces on the button would tie the driving rod in knots, were it not supported by the bore itself - and this can be demonstrated easily enough by taking a 2 or 3 foot length of, say, 1/4" drill rod, clamping one end firmly in a vise, and clamping a visegrip on the other end - mere hand rotation can torque the rod several degrees in either direction.
Barrels made by the other standard processes (cut rifling, broaching and hammer forging) do not suffer from the pitch variations encountered in buttoned barrels. This is not a criticism of buttoned barrels as a class (some of the best barrels made are buttoned), but a commentary on the nature of the process itself.
mhb - Mike

[Bass Ackward]

There's the pitch.

It's a long, ........ high ....... fly ball. It's going .................... going .............. and it's over the wall in dead center field!

Mike has just hit a home run.

[StrawHat]

MHB

Thank you, I am familiar with tooth cut rifling but not button rifling so it took your explanation of button rifling to get my mind wrapped around how the rifling twist could vary.

Okay, now I "think" I understand. Buttons can wobble but teeth have a better chance of coming out right.

Both are accurate, and both are necessary.

For the life of me I could not figure how polishing the barrel would change the twist (pitch) of the rifling.

Thanks again fo rthe explanation.

[uscra112]

Having myself actually measured the lead error of hundreds of leadscrews and ballscrews with a laser interferometer, please believe me that leadscrews CAN and DO vary in pitch, rather more than you'd like to know about if you're depending on one for accurate motion in a machine tool.

Now, being a geezer with little interest in any firearm newer than about 1915, I have to admit that I know little and care less about the button-rifling of barrels. The closest I have come to modern rifling methods was rebuilding some Pratt & Whitney gun drills for Ruger, which caused me to stand next to that hammer-forger gadget they bought. The button process as described does look to have lots and lots of room for pitch error.

I still claim that in PRINCIPLE they are the same, and could recruit a UMich mathematician to do a topological proof thereof if pressed, and that was my point.

But it is doubtless true that there just isn't enough metal in the lands of a barrel to allow removal of enough to do any good if the button process is as hit-or-miss as stated. So mhb wins the point that the lapping a barrel and lapping a leadscrew are not comparable on the basis of expectable outcome.

Do I recall correctly that button rifling was first done for manufacture of military barrels where rate-of-production trumped accuracy?

[mhb]

is variously attributed - the Germans apparently did some experimentation with the process before WW2. Remington's Mike Walker is credited with developing the process and instituting it at Remington during WW2, where it was used to rifle military contract barrels. The decisive factor in instituting this method for military barrel manufacture was the much lower cost in time and money for rifling each barrel, and the fact that quality was sufficient for the military requirements. After the war, Remington and other major industry players further developed the process, and, due to the much lower cost in tooling, independent barrel makers followed - some of them are now quite well-known for best-quality barrels made by buttoning, and, of course, many more 'trade-grade' buttoned barrels are also produced by other makers.
FWIW, instrumented study of rifling pitch variations in buttoned barrels can be a real eye-opener, making one wonder how such barrels could shoot at all well (though, of course, the best match-grade buttoned barrels show minimal variation, or, sometimes, a gaining tendency) - I assure you that no modern lead screw could be produced or accepted which showed a fraction of the pitch variance seen in buttoned barrels - and I'll see your UMich mathematician and raise you my metallurgical engineer/boss at the barrel mine for additional testimony ref: rifle barrels, per-se :->
As I said before, lapping is done in rifle barrels to correct minor surface finish imperfections and, sometimes, small dimensional variations, but will testify that you'd lap yourself (and the barrel) to death before correcting any major boo-boo.
The deep-hole drilling machine in our shop is a Diamond - a more-or-less exact copy of the Pratt and Whitney machines you rebuilt for Ruger. Ruger used to have (may still have) a Pratt and Whitney sine-bar rifling machine (completely rebuilt) and exactly like ours, on display - it had belonged to Bill Atkinson, Ruger's Engineer Emeritus, and one-time partner of Paul Marquart, from whom our operation descends. The lead screw in the deep-hole machine drives the carriage which pushes the drill or pulls the reamer through the bore, and, through gearing, determines the feed rate (about .0003" per revolution on our drill). The rifling machine lead screw drives the carriage which holds and rotates the rifling rod, and, again, through gearing and the sine bar itself, generates the rotational pitch for the rifling rod - between about 1 turn in 5" and perfectly straight, left-or-right hand rotation. Through rack-and-pinion gearing and leather belt-driven pulleys, it also reverses the direction of carriage travel, and, by means of sliding rods pushed and pulled by bosses on the carriage, causes the headstock to index, rotating the barrel blank for rifling of the required number of grooves, and through a cam mechanism, a ratchet and a 'duckbill', raises the cutter in its box to deepen the grooves as rifling progresses.
STRAWHAT: Only buttons depend on their own geometry to rotate themselves through the barrel blank and generate the rifling geometry and pitch - the other processes drive or lead the tooling through the barrel by mechanically generating the desired pitch continuously - in cut rifling and broaching (which both remove metal from the bore to form the rifling), the stresses on the tooling are MUCH less than on the button, and do not tend to torque the cutting tool so much or with so much variation in force as to produce pitch variations in the finished rifling. In hammer forging, the forces are primarily compressive, and the mandrel, which is both rotated mechanically and withdrawn progressively as the barrel is forged and elongated, is several inches long, so significant variations in rifling pitch are hardly likely.
mhb - Mike