Barrel lapping

[bobsmith]

Hi folks. I'm new to the forum but not to guns.
I recently acquired a Ruger 77R in 7MM-08 with serial prefix 75-XXXXX . The original stock has been replaced with a Bell and Carlson synthetic stock. When I received the gun there was some rust in the bore which I cleaned out with a bronze brush. When I look down the bore I can see some very light roughness in the grooves. The rifle's original owner (now deceased) was a good friend of mine and I know it used to shoot 1" 3 shot groups at !00 yards with his selected handloads with the synthetic stock. Now the best I can get is about a 3" group even after using duplicates of the original owner's handloads. I suspect that the roughness in the bore is influencing the rifle's ability to group. I've been wondering if having the barrel lapped will improve the groups. I've done fire lapping in the past and don't like it because it can do nasty things to the chamber throat area. I've read some on lead/rod lapping but have never done it. Does anyone on the forum have any experience in this area? Ruger has quoted me $210 for replacing the barrel but I would like to retain the original barrel as much as possible as it does have a very decent blue job on it.

 

[HSMITH]

Welcome Bob.

The old Rugers, tang safety guns, had really rough bores to begin with but they shot great.

If it were me, I would run a tight jag with some JB Bore Paste through it, just enough to take the sharp edges off the rust pits. 5 or 6 patches, 5 or 6 passes each. Then I would stop cleaning the bore. Shoot a couple boxes and I would wager that the groups will tighten up. Clean and protect the bore with CLP or similar and it should stay shooting good. Often times a pitted bore will shoot a lot better than it has any right to if you leave it alone.

 

[bobsmith]

Yeah, good ol' JB. May be worth a shot. I'll give it a try.

 

[Choctaw56]

The old NRI gunsmithing course I have describes bore lapping thusly: You have to make a special jag by putting a fixture down the bore that you can pour molten lead into to get an exact impression of the lands and grooves. you then load the bore paste on this custom poured jag and work it back and forth in the bore until you have the desired results. You have to clean all the compound out and check your progress as you go. supposedly, just putting bore paste on a tight fitting jag and patch can do more harm than good by actually rounding off the sharp edges of the lands. Hope this helps, but I've never had a bore bad enough to try it on, so I can't attest as to the effectiveness.

 

[dfariswheel]

A more modern approach to bore lapping is "Fire lapping".
There are kits sold to do this.
These kits contain cast lead bullets and a series of fine abrasives.

The abrasives are embedded into the lead bullets by rolling them between steel plates coated with the abrasive.

The bullets are then loaded with reloading equipment and fired through the rifle.

The kits have full instructions, including complete directions on the special, light powder charges to use.

Generally, you start off with a coarser abrasive, then use progressively finer grades.

This is much simpler for the shooter than the difficult and complicated old fashioned hand lapping.

 

[bobsmith]

I tried fire lapping on an M-1 that had a slightly rough Douglas barrel. Using a NECO kit, I followed followed the instructions very closely. I encountered bad fouling problems. At the end of the lapping session, examination revealed that the rifling in the throat area had gotten noticeably shallower with no improvement in accuracy. I changed the barrel. After that experience, I am a little reluctant to try fire lapping again. Therefore the inquiry about hand/leadlapping. I believe it can be done with more control.

 

[sydn]

Put a jag on your cleaning rod and stick it inthe breach so the tip of the jag is about 1/2 " from the muzzle. With the barrel in a vertical position, pour lead in the muzzle. It cools instantly around the jag and you have a lead lap. Lapping compound embeds in the lap, so when you change grit size, melt the lead off the jag and pour a new lap.

 

[bobsmith]

Put a jag on your cleaning rod and stick it inthe breach so the tip of the jag is about 1/2 " from the muzzle. With the barrel in a vertical position, pour lead in the muzzle. It cools instantly around the jag and you have a lead lap. Lapping compound embeds in the lap, so when you change grit size, melt the lead off the jag and pour a new lap.

What grit should I start with? My friend mentioned 220 but I think thats a bit coarse. The pitting in the bore is not really deep but it is visible in good light.

 

[sydn]

Start with 120 on a new barrel, then 220, 400, and finish with bore paste. Use aluminum oxide rather than the sharper silicon carbide. Vary the spot up and down the barrel where you change directions when stroking the lap. I like to use a bit of oil with the grit. You will feel the rough and tight spots smooth out when working the lap. Don't let the lap come completely out of the barrel when stroking unless you mean to melt a new one.
Since this is a used rifle, I think your friend is right about starting with 220.
Don't try to take the pits out. Lapping is for smoothing out rough machine marks from the rifling process or for opening up tight spots in the barrel.

 

[HSMITH]

Make DARN sure any compound you use is non-embedding.

I would start with JB, just enough to knock the sharp edges down and not really change anything. Pits don't mean it won't shoot good, sharp edges do.