About Fire Lapping Barrels
You roll bullets, lead or jacketed, between two hard plates (steel usually) to embed valve grinding compound grit (generic term for lapping compounds you may be familiar with) into the bullets. Then generally with a reduced load, shoot these gritty bullets through the barrel.
It takes the tops off the surface finish inside the barrel going around the inside, and then leaves a finish going WITH the travel of the bullet. It also takes off burrs and sharp edges from the rifling.
And since the grit in the bullets breaks down as it travels down the barrel, the gritty bullets cut less and less as they travel toward the muzzle. Thus you get something of a "choked" bore & groove, and this in itself is a plus to accuracy. Barrels that get progressively tighter toward the muzzle are usually quite accurate.
(As a side note, this is what you get with some "air gauged" barrels... they simply mark the tighter end "muzzle.")
David Tubb also advocates its periodic application to rifles shot a lot, the idea being to smooth up the rough, scorched surface in the throat of the barrel.
However, he also concedes that fire lapping INCREASES THE DIAMETER OF THE THROAT. And given that most all throats are cut larger than they should be, especially from factories, my contention is that while you may make temporary improvement in accuracy fire lapping existing barrels, you also wipe out potentially thousands of rounds of throat life by further enlarging it fire lapping.
I did a little demonstration Memorial Day Weekend, 2002, when visitors were here. I took a standard .30/30 factory barrel, had them look at the throat through the bore scope, then shot only a dozen, 12, rounds of fire lapping ammo through it, and had them look at it through the scope again. A dozen rounds does a heck of a lot of damage to an existing throat unless you are using very fine grits. But you have to run a lot of fine grit through a barrel to make any significant improvement, and by the time you run enough rounds through the barrel, you will still end up ruining an existing throat. So don't do it.
I further contend that fire lapping should only be done when the barrel will subsequently be rechambered to a longer cartridge. In the rechambering process, the old enlarged throat is cut out, and a new throat is cut in fresh rifling ahead of the old one.
Since I cut throat diameters minimum, my throats can be fire lapped a little and you won't hurt anything. The surface in the throat area is crosswise to the travel of the bullet, and several fire lap rounds with fairly fine grit will help smooth it up, but so will 50 to 100 rounds of shooting, especially when a "break in" regimen is followed that does not allow copper to build up in the throat area.
Fire lapping is wonderful.... Lapping is what makes good barrels into benchrest grade barrels in the custom barrel making trade.
But unfortunately, chamber throats are a deep, dark mystery highly obscure to most shooters. Few realize just how much damage they can do to a throat fire lapping it to any degree. It is one step forward and two steps back. The longer lead angle on the ends of the rifling produced by the accelerated wear from lapping may improve accuracy for a time, but in the course of lapping and enlarging the throat diameter, you go backwards.
I have mentioned it before, but will mention it here again. I co-authored the chapter on chamber throats in the 1995 Precision Shooting Annual, and in that same issue there was a MOST EXCELLENT article about fire lapping, in which the author documented how he monitored the growth of the throat during the fire lapping process.... proof positive the throat is enlarged.
The main reason for concern is this. The grit is sharpest and protruding microscopically from the bullet where it first starts moving. In the process of shoving the grit deeper into bullet metal as it engraves the rifling in the throat area, the throat is cut the most of any part of the entire length of the barrel.
In fact, you can do a substantial amount of fire lapping to a barrel to the point the rifling are well rounded at the rear, looking very much like a shot out barrel, but as you look more toward the front end of the barrel, other than being shinier, it shows virtually no change. Just like in the process of shooting out a barrel. They wear out from the breech end forward.
Same with fire lapping. You are doing an accelerated "shooting out" of the barrel, speeding up the process with grit.
Do I recommend it? Highly, but only when the barrel is to be rechambered to a longer cartridge. And when subsequently given a minimum diameter, long lead angle, well centered throat, you will get outstanding accuracy from fire lapping.
I.e.., I discourage fire lapping barrels that are to be shot as they are with the current chamber. You make short term gains, but do long term damage if more than just a very, very few rounds are fired, and at that only with the intent to smooth up the throat a little. If you are trying to smooth the entire length of the barrel, you will ruin the throat before you make much gain on the rest of the barrel. Count on it.
I have been lapping barrels off and on for 20 years, initially machine lapping new barrel blanks. Later after reporting my "playing around" shooting gritted bullets back in the early '80s, the idea took root and others decided to capitalize on it selling expensive kits, but with very little understanding of throats in the first place and the damage the fire lapping does to them.
There is no reason to pay $50 or so for a kit.
You can buy little cans of Clover compound or other lapping compounds for about $10 and lap probably a hundred barrels with it. As for bullets, you don't even have to use bullets. I have drilled appropriate size holes in 1x2" boards, and poured them full of lead to make slugs to roll in grit when I did not have the required bullet mould and wanted to use lead instead of jacketed bullets, which is another area in itself.
All you need then is a couple steel plates or any flat steel surfaces you can roll a bullet between after dabbing lapping compound on them.
I go as coarse as 220 grit if I will be lengthening the chamber much, and may finish no finer than 400 grit.
Some make a big deal about using ultra fine grits and getting a super fine finish, which is well and good, but after you shoot a few rounds of normal ammo, you will 1) improve the finish left from somewhat coarse grit, or 2) degrade the smooth finish to a rougher one by shooting! In the end, you get whatever finish shooting regular ammo produces.
The main objective is to get all of the surface finish in the barrel that contacts the bullet going WITH the travel of the bullet instead of crosswise to it and making the finish more prone to picking up copper.
Done judiciously, fire lapping does wonderful things to barrels. Done blindly, it is good business for barrel makers and the guys selling the kits, but guess what? You lose in the long run more than you gain in the short run.
The only way to do any amount of significant fire lapping is to do it in the original chamber, then rechamber to a longer cartridge that cuts out the old throat damaged by fire lapping.
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