TomG
posted this
05 February 2012
My current project is accurizing a new Ruger Blackhawk stainless 357 mag.
Initial accuracy was dismal. Subsequent inspection revealed a tight spot at the the barrel threads and a loose spot under the front sight ramp in the barrel. Cylinder alignment seemed pretty good using custom made range rods. Cylinder throats were on the tight side for the barrel.
I fire lapped the barrel with very hard cast bullets till the restriction in the threaded area was gone. Use very hard bullets and a very light loads so that the bullet doesn't slug up in the cylinder throat or forcing cone. This improved the loose spot but didn't take it all out. Fortunately, the barrel tightened up at the muzzle again. I lapped all the cylinder throats to one thousandths over the barrel ID. I then bought a 5 degree forcing cone reamer and extended the forcing cone from around 11 deg. to 5 deg. per side.
The lawyer trigger in it had a lot of creep and was pretty heavy. I put a spring kit in it and stoned the sear and notch to get the pull down to a reasonable level. I then put the trigger in my surface grinder and ground the sear notch down till most of the creep was gone. It's got one more trip to the grinder to get it perfect. You want to sneak up on this pretty slowly. You have to keep the notch square to the trigger axis and this almost impossible without using a jig of some sort.
The fire lapping was done with 220 grit initially till most of the tight spot was gone. Then I finished it up with 400 grit. Cylinder throat dia. was matched to the dia. in front of the forcing cone. I would slug the barrel every couple of shot till you get a feeling on how fast it's cutting.
I'm now getting 2 1/2 inch groups at 40 yds. off sand bags and a 4-12 power Burris scope. That's the best I can hold. I'm sure the gun is better than that now. I may make up a Ransom Rest type rest to get better data on this gun and loads.
The moral of this story is to not try to lap with too fine a grit. For it to work, it must cut aggressively. Messing around with fine grits only polishes it up but the tight spots are still there.
Also, you don't want a barrel to be too smooth inside. About 320 to 400 grit is best for lead bullets. You want to have a place for the lube to reside on the surface of the bore. Also, if you have too smooth a surface, you have too much contact of the bullet surface with the barrel surface. This leads to more friction and leading or jacket fouling. If there are tiny peaks and valleys the bullet can ride on the tops of the peaks and lube can reside in the valleys. I recieved this from a top notch barrel maker who's barrels win many matches. It makes sense to me.
The quality of the lapping compounds varries from one manufacturer to another. I've examined several brands under a microscope and found that some looked like floor sweepings and some were very uniform and beautiful to look at through the scope with a strong light. I use Clover silicon carbide exclusively. I've never looked at valve grinding compound and don't know if it will imbed in the parent metal or not. I would stick with non imbedding compound and spend the money to do it right. Most any industrial supply place will have silicon carbide compound in many grits.
Whether fire lapping or lapping by hand, I would not finish with anything finer than 400. One custom barrel maker uses 320 and when it breaks down, gives a slightly finer finish. I've examined many custom barrels with a bore scope and all of them looked like they were finished with something around 320 grit.
I once lapped a barrel down to 1000 grit and it didn't shoot any better. My 357 barrel with the 400 grit finish never shows any leading even at 1500 feet per second loads. .
Just my $0.02 worth,
Tom Gray