Smoothing or Lapping the Bore

If the tight spot that develops during shooting is due to minor roughness in the bore, you can often remove the roughness by rubbing with steel wool.

Take a wad of wool and make it about 1 1/2" across and thick enough that it's a tight fit when you try to use the wool for a cleaning patch. Stuff it down the bore with your ramrod and jag and rub on the place that you want to smooth out. You will want to have this bore wiped clean of oil when you do this so the wool will "cut".

You can use the cleaning jag, or a sheeps wool oiling mop or even an undersized bore brush. I like the jag, if it fits right, because it lets me get all the way to the breech plug. You don't need to unbreech the gun to use steel wool!

Rub a bit and refold the pad and rub some more, etc, until you think you have smoothed up the spot.

Remember that this won't lap the bore but will help smooth out tooling marks or "fuzz" that catch fouling and cause a tight spot during shooting.

Don't make the wool patch fit so tightly that you have to hammer it into the bore; just a good tight fit. The jag doesn't want to be almost bore size either; you want to have enough wool around the jag so it's not badly "pinched" at the rifling lands and only rubs there. You want the wad of wool to have enough thickness so the pressure is sort of even between the lands and the bottoms of the grooves so the whole thing is rubbed evenly. You don't want to just polish the tops of the lands and not the grooves!!

The only proof is to now go out and shoot the gun to see if the tight spot still shows up. Remember that it can take a few dozen shots to re-season the bore again.

If this sounds like a lot of work followed by a lot shooting, well, you're right! But what the heck?

I guess I should make clear the distinction between a "fouling" tight spot and a "size" tight spot just in case there is some confusion: The "fouling" tight spot can be caused by roughness in the bore or other factors such as patch lube. The "size" tight spot is caused by a reduction in the bore size in a local area of the barrel.

Steel wool won't remove enough metal, unless you spend tens of hours rubbing with a very tight wad, to open up a "size" tight spot significantly. A "size" tight spot is effectively worked by abrasive lapping.

Putting a tight cloth patch on your jag, in a lighly oiled barrel, and slowly running it up and down the bore can let you "feel" for tight spots in the bore. Remember that if you have roughness it will snag the patch and feel like a "size" problem sometimes. That's why I recommend that when you think you have found a tight spot with the patch, that you polish the bore with steel wool to remove the roughness that might mislead you. Then if you can still identify a restriction in the bore size, you will need to lap to remove it.

Working in a barrel bore requires that you are attentive to the input coming in through your fingers. Your eyes don't help much. Listen and look through your finger tips.

© 1994 Peter Krogh

Lapping is the process of using abrasives to remove a slight amount of material from a surface in order to smooth it or change it's shape. In our case we want to smooth up the interior surfaces of a rifle or musket barrel. A lap works because it is softer than the surface you want to cut. The abrasive embeds itself in the softer of the two materials and does the cutting until the harder surface is cut away enough that the lap can't apply enough pressure to cut any more. Depending on the amount of cutting desired, you may need to make several laps to acheive the desired results.

We will use the example of a .50 calibre rifle barrel. You will get the idea and can adapt the approximate dimensions given to any size barrel. One key thing: the smaller the bore, the closer the rod must fit in order to not buckle the rod and wear the bore when pushing the lap. Also, the length of the lap will get shorter with a smaller bbl. I would say the lap wants to be four to six diameters long.

Caution: Do not size the rod so closely to the bore that the abrasive grit will jam the rod in the bore. If you keep the rod .03" smaller than the bore you should be alright.

Unbreach the barrel and clean it throughly. Make a steel rod about a foot longer than the barrel, smooth and polished so it won't harm the bore, and of a size to almost fit the bore. On a .50 calibre for example with a bore size of .500, make the rod about .450 dia. In a .30 calibre bore, make the rod about .270 dia.

On one end of this .450 dia. rod, reduce the diameter to about .300 dia. and include some grooves or roughness to grip the lead. Make this reduction about three inches long for a .50 calibre barrel. Just down the rod from the shoulder left by the reduction, make a bit of a groove to wind a string in to make a sort of seal to keep the lead from running down the bore. On the other end of the rod, rig up a "tee" handle with a good ball bearing that will turn freely even with a lot of thrust on it. The handle must be about a foot long so you can get a good grip on it with both hands. You will be putting some effort into this job! Also, you should rig your handle so you can do some pounding with a mallet it the "push" direction. Keep reading...

Clean the barrel and oil it lightly with some light oil like Starrett tool oil. Wind a string in the seal groove and insert the rod into the barrel from the breech end. Make sure the string seal fits tightly so the hot lead won't run out. Push it up until the rod is about 1/2 inch from the muzzle and clamp the whole thing so it will stand vertical with the muzzle up so you can pour in the lead. Heat the end three inches or so of the muzzle with a torch, gently!, until the oil starts to smoke a bit and pour in the lead up to about a half inch from the muzzle. I trust you did have the lead ready! This preheating bit is just like when you cast bullets; if the mold is too cold the slug will wrinkle badly and won't hold the abrasive evenly. If you over heat the barrel you will damage it and the lead will "solder" to the bore. Go slow! It's better to make a few laps until you get the hang of it than risk uneven lapping because of bad wrinkles or "soldering" the rod into your barrel! Minor, tight, wrinkles won't hurt.

Let the mess cool off until you can handle it with your hands comfortably. Grip the barrel in a very sturdy vise on a very sturdy bench in such a way that it won't move with you really pulling on the handle. Sometimes it's better to just 'c' clamp the barrel to the bench top (or a main post in the shop or a big stump!).

Now, gently tap the end of the rod so that you get a half inch of the lap poking out of the muzzle. NOTE: if, during the first four or five passes through the barrel, you let the lap pop out of either end of the barrel, you will have to re-make it. I don't know of any way to re-install it without damaging it and thus damaging your barrel. If, after you have made four or five passes, you remove the lap you can re-insert it if you are very, very, careful.

Trim the end of the lap and sort of bevel it all around a bit. Then carefully tap it out of the barrel some more so it sticks out about two inches. Oil the end of the lap and coat it with some of your abrasive ( more detail later). Pull the lap back into the barrel and pull it down so the lap is almost coming out of the breach. You may have noticed that there were tight or loose places as you pulled the lap the length of the barrel. Note where those are by marking on the barrel with a crayon so you can work the tight places more and even up the diameter.

Push the lap up to the muzzle and poke it out about two inches again. Add, evenly around, some more abrasive. Now push and pull the lap through the barrel, working especially on the tight spots. You can make about 15 or 20 passes before the lap is not effective. You will feel when the lap stops cutting. You can go back to the muzzle two or three times to reload, with the same grade of abrasive, before the lap will be too small to use any more. Use a little abrasive with a lot of oil in the first load and more abrasive for the next loads.

If you have eliminated most of the tight spot(s) with this first lap then the rest of the job is just polishing. When the lap is not effective, pop it out of the muzzle, rip off the old lap, remove the rod, clean the rod and bore, fix the string seal and pour a new lap. If you have done enough with the first abrasive, go to a finer one then a finer one until the bore is as bright as you want it. Do not put a finer grade on a lap loaded for a coarser abrasive; the coarse is still there and you need to make a new one to get the finer stuff to cut.

The abrasive to use with a "rough" bore with lots of tool marks is emery flour. This will cut steel fast so you should be careful to not over do it. Crocus and Rottenstone cut much more slowly and are used for polishing after you have cut out the rough and tight spots. Rouge works well for polishing also. Old fashioned tooth powder, calcuim phosphate, is the ulitmate super polish and you will "pull the handle" a long time with this stuff!

Do not attempt to use any of the so-called "valve lapping" compounds sold in automotive stores! They are absolutely too coarse!! The correct stuff can be obtained from Brownells in Montezuma, Iowa and others. Try industrial supply houses also.

Just be careful to not wear out your barrel by removing too much of that precious steel bore. Don't move the lap fast; speed won't gain you anything and could cause you to "pop out" the lap. These laps cut at any speed. Push and pull in a nice straight line at an even speed for the best results and the least wear on the barrel from the rod. Make damn sure your rod and lap are rotating freely if you are lapping a rifled barrel or you will end up with the strangest rifle in the county! Also, you cannot use the lap in any other barrel. You must make a new one each time.

Be sure to remove all of the abrasive before breeching up again!

© 1995 Peter D. Krogh