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Barrel lapping

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Phil Coffins

69 Cal.
Joined
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This maybe of interest to our budding gunsmiths. This re-rifled barrel has tight spots that I will be reducing and in the process polishing the bore. The un-breached barrel is cleaned and a guide bushing is fitted to the breech end. The rod has a 10/32 cap screw in the end to receive the lead. Masking tape is wrapped on the rod to fit the bore and pulled back about 10” from the muzzle. Lead is melted after warming the muzzle than with the rod pushed forward to about 4” short of the muzzle the lead quickly pored in. This forms a lap to fit the rifling and receive the compound. I’ll start this barrel with common fine valve lapping compound then cast a new lap for finer grits. The tight spots are easily felt and these are four inches in front of the breech to about eight inches from the muzzle. I make an index mark on the lap to insure it’s always put in the bore the same way. Once the lap is coated with grit the tightest part of the barrel is lapped till the resistance is reduced. Then a rod that is close to bore size is put in the muzzle end to touch the lead and given a light tap. This tightens the lap and more lapping is done till another light tap is needed to retighten the lap. As the work progresses the tight spot becomes longer and you notice the loose spots more. Ideally by the time you’re done the bore will feel the same full lenght or a tiny bit tighter at the muzzle on this one. This is not easy or quick but makes a better barrel. Laps from other jobs are in the picture to show what I’m trying to explain.
AE66A307-1951-425E-A659-F7DCA4C0B854 by Oliver Sudden, on Flickr
 
I now have about four hours of lapping on this barrel and the tight spot is about 20” long out of the 30” of barrel. I lap for as long as an old man can or until the barrel is hot enough to be uncomfortable to touch. A few more hours of this then I’ll re-cast the lap and charge the lap with a finer compound. Then I can deal with other issues that has kept me from shooting this rifle. Note how the breech is off center to the bore.
48815BCB-FDB0-4A99-B7A9-9AC4773DA019 by Oliver Sudden, on Flickr
 
As you showed this is not a game for people in a hurry. I often will lap a slight "choke" into barrels that I have had to lap to clean up pits. It takes a lot of time and several changes of the lapping slug to do a good job.
 
A inspection of the lap can tell you more about your barrel. This time I removed the lap by pulling it out of the breech end. There is a burr in one groove that is clearly shown on the lap. This will have to be removed on this project before I’m done to insure the bullet has a clean start into the bore. The over all bore is feeling rather good at this point.
CD6930CB-9538-451B-AC33-3820ABD58BF5 by Oliver Sudden, on Flickr
 
This maybe of interest to our budding gunsmiths. This re-rifled barrel has tight spots that I will be reducing and in the process polishing the bore. The un-breached barrel is cleaned and a guide bushing is fitted to the breech end. The rod has a 10/32 cap screw in the end to receive the lead. Masking tape is wrapped on the rod to fit the bore and pulled back about 10” from the muzzle. Lead is melted after warming the muzzle than with the rod pushed forward to about 4” short of the muzzle the lead quickly pored in. This forms a lap to fit the rifling and receive the compound. I’ll start this barrel with common fine valve lapping compound then cast a new lap for finer grits. The tight spots are easily felt and these are four inches in front of the breech to about eight inches from the muzzle. I make an index mark on the lap to insure it’s always put in the bore the same way. Once the lap is coated with grit the tightest part of the barrel is lapped till the resistance is reduced. Then a rod that is close to bore size is put in the muzzle end to touch the lead and given a light tap. This tightens the lap and more lapping is done till another light tap is needed to retighten the lap. As the work progresses the tight spot becomes longer and you notice the loose spots more. Ideally by the time you’re done the bore will feel the same full lenght or a tiny bit tighter at the muzzle on this one. This is not easy or quick but makes a better barrel. Laps from other jobs are in the picture to show what I’m trying to explain.
AE66A307-1951-425E-A659-F7DCA4C0B854 by Oliver Sudden, on Flickr
Great post! Fine looking shop! Obviously skilled craftsman! Very interesting.
 
I’ve re-bored the chamber and made a new chamber bushing. Got everything lined up and the bore is pretty good so shooting is the test. Started at 50 yards just leaning over the Jeep hood. The fouling shot is the X then the next four clustered into a group. A slight sight adjustment will follow.
IMG_0972 by Oliver Sudden, on Flickr
 
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