Rough Bore

[Rex]

I've had a S&W 686 for 20 years or so and it has always been a barrel leader, doesn't shoot bad just leads. Gas checked bullets work fine but I've got other molds that I like. Read a post by Char-Gar from a ways back and he talked about roughness and machine marks in the barrel. I thought "bologna". Today I found a small magnifying glass that I could look in the back of the barrel with. HOLY MOLY, the lands and groves looked like they had been scratched in with a spring tooth harrow! Now I'm still a little spooky about fire lapping. What are the thoughts about a little fine valve grinding compound on a tight patch and polishing the back of the barrel where the lands begin? Help, hurt or a waste of time?
And my apology to Char-Gar for doubting him!

Thanks a Million
Rex

[Outpost75]

Fire lapping will cure that.

Maybe, maybe not. My experience with fire lapping is that it does most of its work in the barrel forcing cone and cylinder throats, and less as you get farther down the barrel. The tool marks which run in the direction of bullet travel, left by the rifling broach do little harm. The circumferential tool marks across the tops of the lands, left by the gun drill do most harm. These are best removed by charging a turned, brass rod about 0.0015-0.0020" smaller than bore diameter and a bit shorter than the same window, which when clean and oiled is a sliding fit into the bore. Coat this with 600-grit lapping compound and oil, then drive through the bore with a rod and dead-blow lead hammer. It may take 6-10 passes. This also removes "thread choke." Groove diameter will not be affected.

[country gent]

I would recommend something finer than valve grinding compound for this. Also using a patch is liable to round corners of rifling. The best way to smooth the bore would be to make up some lead laps and work it with 320-600 grit lapping compound.
Barrel lapping is a process.
Make 2-3 strokes and rotate one groove repeat thru one rotation and check. This keeps grooves and lands even.
If bore looks good then go to finer grit and repeat.
A bore scope helps a lot to see what you have
Make full even strokes. short strokes or working an partial area makes for loose spots. You want to keep the bore consistent. If you start with short strokes then half length then full length you can actually taper the bore getting smaller from breech to muzzle. Fire lapping does this since the lap only goes thru in one direction once the compound dulls on it trip cutting less as it travels thru the bore.

[RED BEAR]

I would give it a try been my experience that unless you use something rough its not going to hurt i would suggest not going with valve lapping compound and use one of the products made for this.