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Bore lapping
- Thread starter gunnut310
- Start date
Jjmotsch,
You can't reasonably make the leap of faith that what applies to a custom barrel also applying to a mass produced barrel. The former are usually cut-rifled and hand lapped, leaving nothing significant to be gained from further lapping, while the latter are often button-rifled and not stress-relieved before contouring, and therefore frequently have very rough copper-grabbing bores that are not uniform in diameter all the way through. You have to be careful about extrapolating preferences from one kind of critter to another, as it often doesn't work out.
When I got my first Garand from the DCM back in the 80's, it shot great for rounds 1 through 40, then the groups opened up substantially. It always cost me at least 8 points in the last ten rounds of the 50 round National Match Course, and I had to consider it unusable for an 80 round NRA Highpower match. The problem was rapid and significant copper accumulation in front of the throat that took several hours of cleaning with Sweet's 7.62 (this was before the modern chelating cleaners were available) to get down to bare metal again. After firelapping, I was able to shoot it through three matches on three consecutive days without cleaning, just to see if it could be done. Accuracy was unaffected by the firelapping, and in that case, wasn't even the object of the firelapping exercise in the first place.
You can't reasonably make the leap of faith that what applies to a custom barrel also applying to a mass produced barrel. The former are usually cut-rifled and hand lapped, leaving nothing significant to be gained from further lapping, while the latter are often button-rifled and not stress-relieved before contouring, and therefore frequently have very rough copper-grabbing bores that are not uniform in diameter all the way through. You have to be careful about extrapolating preferences from one kind of critter to another, as it often doesn't work out.
When I got my first Garand from the DCM back in the 80's, it shot great for rounds 1 through 40, then the groups opened up substantially. It always cost me at least 8 points in the last ten rounds of the 50 round National Match Course, and I had to consider it unusable for an 80 round NRA Highpower match. The problem was rapid and significant copper accumulation in front of the throat that took several hours of cleaning with Sweet's 7.62 (this was before the modern chelating cleaners were available) to get down to bare metal again. After firelapping, I was able to shoot it through three matches on three consecutive days without cleaning, just to see if it could be done. Accuracy was unaffected by the firelapping, and in that case, wasn't even the object of the firelapping exercise in the first place.
Unclenick said:HiBC has it right. Barrels and barrel making processes are not equal.
***
I confess that a lot of that was so far beyond my experience that I don't completely understand it, but reading that sort of post is why I come here to read for pleasure.
If I may piggyback with a different barrel break-in question: Green Mountain has a rimfire break-in schedule they send along with a rimfire barrel. Can a rimfire barrel be firelapped?
I don't see how a wax covered lead bullet could possibly take any rough spots off a steel barrel, but I followed the Green Mountain schedule anyway. Yes, the barrel did get more accurate after their 100 round schedule. It also became even more accurate over the next thousand rounds over which it may have gotten nothing more than a dry patch every 100 or 150 rounds. The fellow who has it now uses nothing but a boresnake, and it performs at least as well as when I had it.
Does the use and cleaning of a 22lr barrel take off any sharp edges or are we just smoothing the surface by depositing lead and wax into low spots, like an asphalt patch in a pothole?
Thanks
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Lead certainly is not going to wear the steel in,but that may not be the point.
Folks who are skilled in knife sharpening know what "the burr" is.When you hold your angle and stone the blade to the point where you have met the edge,the steel will feather to a very thin burr.Its not part of your blade bevel.It folds,breaks,and is generally not desirable.Removing that burr is a step in the process. There are various ways to do it.
Some folks use a wine cork,some hard felt,some newspaper.Some just use a different stone stroke.
The steel of the knife will cut a lot of wine cork,but the cork will break off the burr.
IMO,a lot of what "break in" is about is NOT the bore,but the inevitable little burrs of chambering.
With your 22 barrel,maybe,"If it ain't broke,don't fix it" is a plan.
If you feel compelled to experiment,A 22 should be easy.
I suggest looking to DME mold supply ot Gesswein. Lookat grit charts for a grade. I used #9.I don't recall the micron. I suggest about 900 to 1200 grit diamond mold polishing compound. It comes in a little syringe.
The full dia of a 22 lr bullet is the same as the case.Smear a little diamond on some glass .Use another piece of glass to roll a few bullets through the diamond compound,embedding the diamond in the lead.Try not to charge the brass with grit.Wipe off the excess.
I,myself,would not want the bore quite dry,but not wet,either.Off the top of my head,poking a very loose patch or maybe a nylon brush with just a little Hoppes on it would be good.Not wet,just a drop or two.
You can have your fun with not much chance of doing harm.I'd slow down at about 15 or 20 rounds. You actually might do some good on a Green Mountain.
But I will say again,trying to improve on what the barrel maker sent you is a separate issue from removing the micro-burrs left by the chamber reamer.
Folks who are skilled in knife sharpening know what "the burr" is.When you hold your angle and stone the blade to the point where you have met the edge,the steel will feather to a very thin burr.Its not part of your blade bevel.It folds,breaks,and is generally not desirable.Removing that burr is a step in the process. There are various ways to do it.
Some folks use a wine cork,some hard felt,some newspaper.Some just use a different stone stroke.
The steel of the knife will cut a lot of wine cork,but the cork will break off the burr.
IMO,a lot of what "break in" is about is NOT the bore,but the inevitable little burrs of chambering.
With your 22 barrel,maybe,"If it ain't broke,don't fix it" is a plan.
If you feel compelled to experiment,A 22 should be easy.
I suggest looking to DME mold supply ot Gesswein. Lookat grit charts for a grade. I used #9.I don't recall the micron. I suggest about 900 to 1200 grit diamond mold polishing compound. It comes in a little syringe.
The full dia of a 22 lr bullet is the same as the case.Smear a little diamond on some glass .Use another piece of glass to roll a few bullets through the diamond compound,embedding the diamond in the lead.Try not to charge the brass with grit.Wipe off the excess.
I,myself,would not want the bore quite dry,but not wet,either.Off the top of my head,poking a very loose patch or maybe a nylon brush with just a little Hoppes on it would be good.Not wet,just a drop or two.
You can have your fun with not much chance of doing harm.I'd slow down at about 15 or 20 rounds. You actually might do some good on a Green Mountain.
But I will say again,trying to improve on what the barrel maker sent you is a separate issue from removing the micro-burrs left by the chamber reamer.
HiBC said:Smear a little diamond on some glass .Use another piece of glass to roll a few bullets through the diamond compound,embedding the diamond in the lead.Try not to charge the brass with grit.Wipe off the excess.
I do recall an acquaintance doing that a couple of decades ago. He thought it helped.
Why is it necessary or desirable to use a bullet fired down the barrel to do the polishing? Wouldn't the polishing process be easier to control by applying the compound to a cylindrical pad that is worked through the barrel by hand?
Sure. I think the book was "Modern Gunsmithing"by Clyde Baker.
He described a process of lapping a bore. A tapered brass wood screw is fitted to a cleaning rod Inside the bore,molten lead is cast around the screw.
Now you have a lead lap that is true to dimension and form with the rifling.Turning the tapered screw will up size the lap
Uniformity is a desired outcome,along with smooth.
Used with technique,a slight "squeeze bore" can be lapped in.
The firelapping with the lead bullet works in similar fashion. The lead swages to conform to the bore. Any tight spots will swage the lead down,but the cut will be most aggressive at the tight spot.
The bullet will maintain a uniform FORM and DIMENSION.
That's not true with felt or mop or patch.It changes.What control do you have?
He described a process of lapping a bore. A tapered brass wood screw is fitted to a cleaning rod Inside the bore,molten lead is cast around the screw.
Now you have a lead lap that is true to dimension and form with the rifling.Turning the tapered screw will up size the lap
Uniformity is a desired outcome,along with smooth.
Used with technique,a slight "squeeze bore" can be lapped in.
The firelapping with the lead bullet works in similar fashion. The lead swages to conform to the bore. Any tight spots will swage the lead down,but the cut will be most aggressive at the tight spot.
The bullet will maintain a uniform FORM and DIMENSION.
That's not true with felt or mop or patch.It changes.What control do you have?
The bullet will maintain a uniform FORM and DIMENSION.
That's not true with felt or mop or patch.It changes.
I see.
Thanks for the explanation. I'm not sure I'll try it unless dissatisfaction with a barrel coincides a need for a project, but at least now I get the principle behind the practice.
Zukiphile,
NECO reported firelapping .22 rimfire barrels. They start with their 400 grit lab grade silicone carbide and polish with 800 grit, and then 1200 grit. They do not have special ammunition for this. They just impress standard velocity cartridge bullets with the abrasive, IIRC. You could go to a slower pistol match bullet, I suppose, or even a CCI CB Long (so the brass is the same length). They reported the following result:
I think HiBC is right that break-in of a rimfire barrel is about stropping the surface to clear wire edges and burrs and there is often a bit of loose blue and other tiny bits of manufacturing detritus left behind to clear. If you firelap, as NECO did, that will do actual material removal, albeit a very tiny bit with such fine abrasives. I would expect something on the order of a tenth of a thousandth.
I would take a look at Bore Tech Rimfire Blend as a cleaner.
NECO reported firelapping .22 rimfire barrels. They start with their 400 grit lab grade silicone carbide and polish with 800 grit, and then 1200 grit. They do not have special ammunition for this. They just impress standard velocity cartridge bullets with the abrasive, IIRC. You could go to a slower pistol match bullet, I suppose, or even a CCI CB Long (so the brass is the same length). They reported the following result:
Several years ago, NECO conducted a scientific study and analysis of twenty-seven .22-caliber rifles belonging to the University of San Francisco Shooting Team. The overall result, as was subsequently reported in a technical paper presented to the National Defense Preparedness Association, was a 15% average decrease in group size. In view of the fact that these were considered to be match-grade barrels prior to lapping, this is a substantial improvement.
I think HiBC is right that break-in of a rimfire barrel is about stropping the surface to clear wire edges and burrs and there is often a bit of loose blue and other tiny bits of manufacturing detritus left behind to clear. If you firelap, as NECO did, that will do actual material removal, albeit a very tiny bit with such fine abrasives. I would expect something on the order of a tenth of a thousandth.
I would take a look at Bore Tech Rimfire Blend as a cleaner.
One of the dangers of explaining something to a neophyte is that you will prompt additional questions. I've done a bit of over lunch research on this, but still have some questions.
If I were going to impress an abrasive into the wax covering a bullet, I would want to use the abrasive in its dry form so that what ever it is suspended in doesn't just carry the abrasive away from the bullet. It that wrong?
I read that for some of these abrasives (silicon carbide, but cerium and aluminium oxide as well) that they should be suspended in water or oil. This would seem to this layman to make impressing the abrasive into wax more difficult. Or could one just smear a bit of the stuff around the fatest part of the bullet prior to chambering?
Thanks
If I were going to impress an abrasive into the wax covering a bullet, I would want to use the abrasive in its dry form so that what ever it is suspended in doesn't just carry the abrasive away from the bullet. It that wrong?
I read that for some of these abrasives (silicon carbide, but cerium and aluminium oxide as well) that they should be suspended in water or oil. This would seem to this layman to make impressing the abrasive into wax more difficult. Or could one just smear a bit of the stuff around the fatest part of the bullet prior to chambering?
Thanks
Silicone carbide is sharp and will embed in the lead right through lube. The oil it is suspended in may help with lubrication, but mainly it will keep abrasive powder from getting loose and all over things you don't want it on. I was interested in telescope mirror making as a youngster, and contamination of the dry grit was a real issue you had to keep on top of. Don't worry about cerium oxide. That's for a final mirror finish. You don't need that in a barrel, as shooting into powder fouling will roughen the surface a little and undo it anyway. As a result, a super smooth finish becomes another thing that has to break in over time. I think NECO's 1200 grit finest grade is as far as you want to go. Aluminum oxide can be used, but its rounder grain shape cuts much more slowly. Be prepared to need three to five times as many lapping rounds with it.
The way the NECO kit works is you roll the bullet against abrasive between two steel flats, and that embeds it. I did a lot of more aggressive firelapping on old military barrels using jacketed bullets, and even the copper picks it up. I could wipe the bullets dry before seating and still see and feel the abrasive.
The main difference with .22 rimfire is you are not embedding the bullet before seating it. The heel makes that possible to do, but I put a layer of masking tape around my cases to keep the abrasive off the brass. Probably an overkill precaution.
The way the NECO kit works is you roll the bullet against abrasive between two steel flats, and that embeds it. I did a lot of more aggressive firelapping on old military barrels using jacketed bullets, and even the copper picks it up. I could wipe the bullets dry before seating and still see and feel the abrasive.
The main difference with .22 rimfire is you are not embedding the bullet before seating it. The heel makes that possible to do, but I put a layer of masking tape around my cases to keep the abrasive off the brass. Probably an overkill precaution.