BORE LAPPING
by TOM MOORE
The following comments, observations and ramblings are provided as a record of my experience in bore-lapping, more specifically fire-lapping, a S&W Model 625-6 Mountain Gun in .45 Colt caliber. They are offered as information and hopefully will be of help to anyone who decides to travel this path.
First a description of the gun and reasons I decided to lap its bore. I purchased the gun second hand and after cleaning, polishing up the internal lockwork and purchasing a set of Craig Spegal "Boot Grips" I proceeded to work up some mild loads using Winchester large pistol primers and W231 powder. I loaded in batches of fifty rounds starting with 5.5 gr up to 7.5 gr using a commercial cast 250 gr swc cast bullet sized to 0.452" with a BHN of 16 by Mid-Kansas Bullets. After fifty rounds of any of the mentioned loads there was severe leading in the forcing cone and in the breach end of the barrel. Leading was also in the cylinder throats and chambers and on the cylinder face. It almost seemed like the only place there was no lead was on those boot grips!
Cylinder throats measured from 0.4520" to 0.4525" and a slug pushed through the barrel would just slip through each throat so this size and bore/throat relationship seemed ok. Barrel to cylinder gap was 0.005" on one side and 0.007" on the other. Knowing this wasn’t what it should be I sent the gun back to S&W for re-work. They corrected this to a constant 0.003" on both sides solving the problem of leading on the cylinder face. However, the other leading problems still were present. I tried changing bullet alloy by using bullets with a BHN of 11 thinking maybe the charge was too light for the alloy hardness. This helped somewhat with the bore leading but the forcing cone leading remained unchanged.
The leading in the forcing cone was in the outer portion near the barrel face. Since the bullets never made contact in this area, the only way I could explain this leading was that it was caused either by vaporization of the bullet base or by blowby due to the bullets not "slugging up" to the chamber throats or bore. Since W231 is a fairly fast powder, I reasoned that maybe by using a slower burning powder this leading would be reduced. I had some IMR4227 on hand I use for a 22 K-Hornet which is substantially slower than the W231. I loaded fifty rounds with 16.5 gr of the IMR4227 using the swc’s cast with the 16 BHN alloy…..no leading…..absolutely none in the forcing cone.
I love it when a plan comes together…..however, the plan was not perfect. The bore looked like it was lined with lead, especially in the breach end but to some extend it was full length. So, by changing powders I had only moved the location of the leading.
At this point in my relationship with this gun I remember thinking back to the reasons why I bought it……for the life of me I didn’t remember that one of those reasons was the fact that I wanted to spend a lot of time cleaning and removing lead from its barrel….I like the smell of Hopps No. 9 just as well as the next guy but enough is enough. Also, I really like the gun and decided that if I could fix or at least minimize this problem I was willing to spend the time and effort.
I knew that it is not that unusual for revolvers to have a bore restriction where the barrel is threaded into the frame due to torquing the barrel in place. I had a catalog from LBT and reading through it noticed that Mr. Smith had a listing for what he called "push through slugs". These are lead slugs with a "tail" molded in place so that they can be threaded onto the end of a cleaning rod and pushed through the entire barrel giving a "feel" to the condition and tightness of the bore. I purchased some of these in .45 caliber.
Starting one at the muzzle it was quickly apparent that there was a slight tightness just past the muzzle end with the bore freeing up until the slug came to the start of the threads where again it tightened. Continued pushing of the slug showed the bore again freeing up for a short distance and then about a half inch from the breach end the slug froze and would not go any further. It was at this point that I determined that I didn’t wish to chase any more lead rabbits until I had a bore that was at least the same diameter at both ends. LBT also markets a bore lap abrasive compound which is to be impregnated into the driving bands of cast bullets which are then fired using a low velocity load. I purchased a bottle of this abrasive together with a couple of lap plates. The lap plates are steel plates on which some of the lapping compound is smeared. A bullet is then placed on its side on one of the plates and the other plate is positioned on the top side and the bullet is rolled back and forth between the plates causing the compound to be embedded into the driving bands.
Following Mr. Smith’s instructions that the powder charge should be only large enough so the bullet exits the barrel, I loaded six rounds using a 22 short case full of Bullseye. Also per his instructions, I used bullets with a BHN (measured with a LBT hardness tester) of 13 (he recommends 12 to 14) to which I embedded the compound until the driving bands were a dark gray color. These were 0.454" in diameter to insure full contact with the cylinder throats and engagement with the barrel grooves. I was careful to fire the same number of rounds through each chamber so that each throat would receive the same amount of abrasive action. For the first eighteen rounds most of the bullets stuck somewhere in the bore although some would exit. Toward the end of this quantity they started sticking near the muzzle, sometimes with about half of the length extending past the muzzle. For a trap I used a stack of newspapers about eighteen inches deep. Bullets that did exit bounced off the newspapers without any penetration. Distance was about fifteen feet. At the end of eighteen rounds I thoroughly cleaned the bore and chambers and sprayed a little WD40 down the bore. I reinserted the bore slug checking again for the tight spots. I could not feel any difference from what I had before.
I continued this process, stopping to clean after twelve to eighteen rounds and reinserting the bore slug. After the total number of rounds fired was around forty eight , the bullets stopped sticking in the barrel and started penetrating the newspapers by about a half an inch. This was an indication to me that the bore was starting to be opened up. This was confirmed when the bore slug was inserted and a difference could now be felt at the restriction spots. However, the slug still would not pass through the bore at the breach end. Also when cleaning I noticed that there was some slight leading in the bore indicating the velocity was getting to high. The evidence of the smoothing action of the abrasive could now be seen in the bore and the surface finish was beginning to be much smoother and scratches were beginning to be removed.
To minimize the possibility of leading, I reduced the powder charge from a full 22 short case to one about ¾ full, then about ½ full and finally ended up using a charge which was slightly less than 1/2 of the case filled with powder. Leading will prevent the abrasive from cutting so it cannot be tolerated.
After sixty rounds total, I started cleaning after each group of six followed by both visual and slugging inspections. I did not want to open up the bore and throats too much and wasn’t sure of the rate of metal removal.
After one hundred and eight total number of rounds, the bore slug would pass entirely though the bore without restriction. At this point no additional lap bullets were fired. The bore was hand polished using a tight fitting patch saturated with JB compound from Brownells. The patch was passed through the entire length of the bore about fifty times followed by a through cleaning. This treatment was also applied to each cylinder throat using a Lewis Lead removal tool impregnated with the JB compound. Again following Mr. Smith’s recommendation I coated the "new" bore with S&W Friction Block lube. This lube, according to Mr. Smith, penetrates the pores of the metal and further minimizes leading. I also applied the lube to each cylinder throat. I now had a bore with no restrictions and a surface finish that visually looked much smoother than when I started. Additionally the cylinder throats had a much smoother finish.
As far as size, the cylinder throats final sizes were in the range of 0.4528/0.4534".
The barrel bore size was 0.4525" as best I could measure the slug (remember S&W has five lands and grooves).
A slug inserted into the bore at the muzzle would pass through each cylinder throat. So the correct relationship between bore and throat sizes had been maintained.
So, to summarize, from a mechanical standpoint, the process produced a bore and cylinder throats that are much smoother than when started and it removed the restrictions that were in the bore at the muzzle and breach ends.
The next step was to see if functionally and from a practical standpoint I had ended up with something better than when I started and to determine if it was worth the effort.
I loaded fifty rounds with 6.0 gr of W231 and 255 gr swc’s sized to 0.452" with a BHN of 14 and went to the range. These rounds were fired and the gun brought home for cleaning. After swabbing bore and chambers with Hopp’s and running a dry patch thru each, a visual inspection again revealed leading in the throats and bore at the breach end. Also, the leading in the forcing cone outer diameter was again present. This was discourging to say the least but I sort of expected the forcing cone buildup with this powder since the lapping should not have had any effect on this. However, after pushing a worn bore brush wrapped with some copper scrubbing pad strands thru the bore, the leading was gone. The lapping defintly reduced the effort to remove the leading.
Since before the lapping operation I had experienced leading problems with both W231 and IMR4227, although at different locations in the barrel/forcing cone, I decided to test a load using a powder in between these two in burning rate. Using Universal Clays I loaded a fifty round batch of 7.5 gr and the 255gr SWC made from the 16 BHN alloy and took off to the range.
Again, after firing these rounds and scrubbing the bore and throats with Hopp’s and dry patching, there was leading in breach end of the bore but almost none in the throats. There was only a small amount in the forcing cone. Again also, the leading came out with only minimal effort with the bore brush wrapper with copper scrubbing pad strands.
To obtain another data point with a slower powder, I loaded another fifty round batch with 17 gr of the IMR4227 and the same bullet. Bore leading was similar to that obtained prior to lapping though not quite as severe. There was no leading in the forcing cone. Minimal effort was required to remove the leading.
As I stated at the start of this write-up, this is a record of my experience with "fire lapping" one S&W revolver. I am by no means an expert about the process or what results may occur with some other gun or material. My personal opinion is that the process should be of some benefit to most if not all production guns if for no other reason than it smoothes the bore and minimizes the effort required for cleaning. It should also have the effect of producing somewhat higher velocities for a given load…..I don’t have a chronograph so I can’t confirm this.
For this case, it did not eliminate the bore leading but my opinion is that I simply have not found the right load combination for this particular gun. My, my…..guess I’ll just have to keep on loading and shooting…..tough assignment….hope I’m up to the task.
Each person will have to evaluate for themselves whether it is worthwhile to spend the time and effort. As for myself will I use the process on other guns……probably, since the engineer in me says that rough surface finishes and/or uneven sizes in bores just shouldn’t be allowed.
Since the start of this project the materials and information for the fire lapping operation are no longer available from LBT….or at least that is my understanding. Similar materials are available from MidWay and detailed information about the process is I believe still available from Beartooth Bullets.
Midway
1-800-992-8312
www.midwayusa.com
Beartooth Bullets
P.O. Box 491
Dover, Id. 83825