Bore lapping

[zukiphile]

Originally Posted by HiBC

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?

[HiBC]

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?

[zukiphile]

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.

[Unclenick]

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:

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.

[zukiphile]

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

[Unclenick]

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.