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I bought a new GP100 .357 stainless with 4 inch barrel.

I hand-lapped the lands of the bore. The grooves were not lapped.

I'll see if I can find my brief write-up of the results, and I'll post 'em here.

FJ Lee Denver Co

46 Posts
Originally written June 11, 2007

I've decided to manually lap the lands of the bore of my .357 GP100. It's pretty rough looking in there, in certain areas, and examination of recovered jacketed bullets shows some snagging and tearing of the jacket. Fouling in 2-3 small areas seems to build up fast, and is VERY difficult to remove, even with ammonia and BRASSO.

I've done this before on rifles, but never on a handgun. I'm hoping that the overall "mechanics" of the procedure will be easier on a handgun, since the barrel is so much shorter.

My goal is less copper fouling, and easier bore cleaning. If my groups shrink a bit...frosting on the cake!!!

LAPPING part #1

I received the laps from MSC in record time......I ordered on Monday, they were in my hands Wednesday, with no special shipping involved. And the order was correctly filled!!

I bought 3 laps. One lap for each grit size that I was gonna use. I said a week or so ago, I was gonna lap the lands in the bore of my Stainless 4
inch .357 GP100. Which I did, and which I'll describe here.

First I polished and lubed the "adjuster" for the laps. The adjuster needs to be velvet-like smooth so that you can maintain diametric control of the lap.

Then I did a touchy-feely on each lap to make sure there were no tits or hunks of foreign stuff protruding. Then spun 'em in a lathe and very lightly applied some worn out Scotch-brite pad. Then an eye loupe exam, particularly along the expansion slots. Washed the laps. Air pressure dry. All looked good.

I mounted the Ruger in a fixture I made so barrel was pointed upward, vertically. Carefully inserted the unexpanded dry lap into the bore. Pulled it out. Turned adjuster slightly 'til the lap was about .3460 inch in dia. Now there was no slop, but no appreciable drag on the dry lap either. Miked the lap. "Unexpanded" it, withdrew it and charged it with lapping compound. This initial lapping used 220 grit silicone carbide lapping compound. Expanded it again til it miked at almost the previous diameter.

Oiled bore.

Insert "charged" lap into bore, inserting it all the way thru til it comes out at rear, thru the forcing cone. Turn the adjuster (expanding the lap) just a tiny amount. It now is a slightly snug fit (but movable) in the bore, and any "peaks", "tits" or other constrictions will be immediately apparent.

In my case, I had 1 BIG peak, and 2-3 smaller peaks, and an annular constriction where barrel fits into frame. About half way down the bore was another annular constriction. The big peak came off immediately, almost like a "chip" of metal breaking off. The smaller peaks smoothed off with a bit of "local" action. The 2 constrictions took longer. I had to "re-charge" the lap a couple of times. Finally, I could feel it just starting to "come in".......fairly even resistance for the full length of the bore. At no time did I allow the expanded lap to exit the bore, at the muzzle. I stayed slightly back from the muzzle. A little bit of "choke" at the muzzle is fine with me, as long as there is no prominent "snags" of metal extant.

As soon as I felt the complete length of bore "coming in".......I turned adjuster back (unexpanded it)........and withdrew it from bore. Clean bore 1000% clean. Remove all possible traces of the lapping compound.

Charged another lap with a finer compound........garnet compound at 600 grit. Repeated previous procedure, tho now it was much much smoother. Almost cannot feel any trace of the previous peaks or constrictions. This is more of a polishing regimen now. Recharged the lap once. Used lots of oil. As soon as I feel it "coming in"......same routine and pull it out. Clean the bore.

Now is the final polish, with 1000 grit garnet. Now its real close to a silky hydraulic smoothness, full stroke. Same routine.......time for cleanup.

Hospital clean-up. Wipe, flush, brush, whatever, to get all the compound out of the bore.
I sometimes use KROIL after the bore is clean, tho I didn't this time. I used Hoppes. Then I went in there with an undersized bore bristle-brush, wrapped with a patch, soaked with BRASSO. Polished the Dickens out of it. And it feels sooooooo smooth.

Washed out the BRASSO. Final rinse with Brake Cleaner, and a blast from an air hose. This bore was shiny before the lapping........ But now it's much shinier!! Shinier than the gleam in my Daddy's eye almost 65 years ago......

OK.....this post is probably too lengthy. So the results of my work will be in another post. The Ruger has been shot approx. 160 times since lapping, so I do have results to talk about.


I previously posted my procedure for lapping the lands in the bore of my GP100.

This is a follow up.......with some initial impressions and some of the results of that lapping procedure..

I went camping and shooting after the lapping job, and put approx. 160 rounds thru the gun. This AFTER the lapping job.

I fired 4-5 foulers, and then started on my paper targets. Five shot groups at 7 yards. I fired 14 each of those 5 shot groups over the course of 2 days. All with jacketed 125 grain bullets. The 14 targets are in my possession.

Doing 14 targets does NOT indicate an arbitrary number of targets. I did 14 targets cuz on my previous (pre-lapping) shoot session I had done 14 each 5 shot groups, starting with a clean bore that had been fouled with 4-5 foulers. All with JHP bullets. Saved those targets. Trying to make the pre-lap and post-lap comparo somewhat apples/apples.

Did the 14 targets (70 shots total)......and then did some informal plinking and playing, fought off an attack by an enraged pack of pine-cones, put down an errant pop can, etc. Bringing the total number of shots fired from my newly lapped Ruger to about 160. All jacketed.

Set the gun aside for other diversions. When I got home from camping, I was eager to see how the bore would clean up after having endured 160 jacketed 125 grain bullets. Prior to the lapping, the bore would have a visible copper "wash", especially on the lands. With 2-3 "localized" spots of jacket material sorta clumped up.....and VERY hard to remove. Very, very, very hard to dissolve and scrub out those clumps of jacket fouling!!!

After lapping, and after shooting the 160 rounds, there _IS_ once more a copper wash/streaking visible. No clumps tho. Alternating Hoppes and ammonia, I knew immediately that clean up had been made much much easier. No "elbow grease" required. Just a few swipes with the 2 chemicals, and that bore was spotless. And the smoothness with which the patches passed thru the bore felt soooooo good.

So my goal of a less jaggedy, raggedy, peaks 'n valley's bore; and easier removal of jacket fouling, has surely been met. Prior to lapping there were also striations in the bore, more on the lands than in the grooves. Those striations are still there, and visible when the bore is completely dry. They are _NOW_ not as pronounced tho, and they are NOT visible if there is a film of Hoppes in there. I suppose where before I had "simple, ugly sharp-edged striations".........I now have "polished ugly striations". The lapping just polished off some amount of the higher edges of the striations..........either actually cutting off the high edges, or at least "rounding off" the sharpness. In any case, it looks lots better. I think I can now happily shoot more jacketed rounds between cleaning sessions, and the cleaning sessions themselves will be much less labor intensive.

This is another fairly long I'll end it. Part 3 (the final chapter) will talk of what I observed at the target after shooting with the lapped bore.
What did the lapping do to the grouping ability of the GP100?


Enhanced grouping capability was not my initial goal. If it happened that the gun DID shoot tighter groups........Hey!!......I'll take it!!

I got a surprise.......the grouping ability has apparently been enhanced. I'm not a statistician. I'm talking "averages"....and that can get a person in trouble, especially if you're in the company of a statistician. However, I am happy that my "average" group shrunk, rather than needing to report that my "average" group grew larger by X.X % after lapping!!

On average, my groups were 5.6% smaller, center-to-center, after lapping. This thru the course of firing 70 shots........14 each targets of 5 shots each. 70 shots/14 targets BEFORE lapping, and 70 shots/14 targets after lapping. From a bag rest, and SA. Not a large enough "sample" to be statistically valid...? I wouldn't know.......see above!

And in the course of shooting those 14 "after lap" targets, I lowered my personal "best ever" .357 GP100 5 shot center-to- center group size by a substantial amount. Prior to lapping, my best group in my _young_ revolver career had been .595 inch c to c. After lapping, I soon popped a .505" c to c. 5 shot group. Believe me, I tried to manipulate those calipers to get that measurement under 1/2 go!! My wife suggested putting the target in the clothes dryer!!!

FJ Lee Denver CO

186 Posts
I once had a gunsmith tell me that I should lap a barrel using a cleaning patch soaked in Flitz and wraped around a nylon brush smaller than barrel. Then you run it thru the barrel until your hand falls off. According to this smith it will be super accurate after this. After this you never use a brush on the barrel again, just the Flitz. I have never tried this, not having the time or patience. Just wondering what the general consensus was.

Retired Moderator & Gunsmith
16,181 Posts
As a gunsmith, customers could not afford to have me lap their barrels because of the time involved. I've seen lapping jobs others have done .... most times, the bore came out more damaged than improved. The sharp edges of the lands are what bite into the bullet to create spin and form a seal between the bullet and bore. When the edges get rounded (which is one of the first things to show up), you went too far. As the bullet's seal is breached, velocity will drop, sometimes dramatically. Accuracy won't change much; maybe a little better, maybe a little worse after lapping, however fouling usually does improve.

When you see bad bores, it's often caused by one of two things. The factory used a dull cutter which resulted in chatter marks in the bore or .... The hardness of the barrel was too soft so when it was threaded into the frame, the barrel yielded instead of the frame. The frame has to be more elastic than the barrel or you will end up with a slight bore obstruction where the threads from the frame impinged. Both of the above conditions are covered by Ruger (or most other companies) as defective material. As such, they will either replace the barrel or replace the entire revolver if the frame was at fault.

In general, stainless steel barrels have a rougher bore than carbon steel barrels. A couple thousand rounds later, both barrels will smooth up but the stainless barrel will still be rougher. In my professional opinion ... no big deal, just shoot the gun and don't worry about it. If the bore condition is so bad that you think it needs to be lapped, send it back to Ruger and have them replace the barrel. If you do lap your barrel, you can forget about the factory replacing it for free.

Fire lapping is the fastest way I know of to demo a barrel. Lots of people do it and lots of them swear it was successful. Professional will tell you not to.

Gale McMillan was one of my heroes. I bought several of his barrels to use in custom rifles that I built. He personally taught me a lot about rifle barrels to include break-in and lapping. He wasn't into handguns much but he was one of the most respected rifle barrel makers in the world as well as being one of the best bench rest shooters in history. Unfortunately, Gale passed away but his legends live on. Google "Gale McMillan" and see what he had to say about lapping a barrel.

13 Posts
have fire lapped using a cast boolit rolled in rubbing compound and fired really slowly through a dark bored Nagant and steyr carbine, really shinned and smoothed them up. But I am to affraid to try this on a new gun. I found no measurable difference in bore diameter after doing this but it really did slick them up. Have read that hand lapping can cause problems if not done correctly?
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