Fire In The Hole
September 23, 2010
Firelapping bores is a simple job that can boost accuracy.
Rifle barrels are mysterious creatures that are often reluctant to reveal their secrets or do what we expect--or hope--they should. Over the years, shooters have sought expedient methods to improve the accuracy of standard barrels. One of those methods is a process called firelapping, where bullets embedded with abrasive compounds of varying degrees of abrasiveness are shot to smooth out imperfections in the bore.
The process was devised by the late Roger Johnson and tested with the help of the University of San Francisco rifle team some years ago. They found that the accuracy of their .22 target rifles was improved and the guns were also much easier to clean.
Over the years I have used the procedure several times and always found it to be beneficial, so when a new Savage tactical rifle didn't meet my expectations--realistic or not--I pondered what to do. A look through a Hawkeye bore scope revealed some tool-mark scoring on the bore that I thought could be smoothed out by firelapping.
First, a little background. All barrels begin life as a solid rod of steel that is cut to the desired length and then deep-hole-drilled to start the process. Deep-hole drilling isn't like what you and I do with the little twist drill we got at the hardware store. Instead it has a single cutting edge, and lubricant is pumped through a hole in the center of the drill to flush out the chips. The diameter will be smaller than the finished bore, and the most important thing is to have a hole that is straight down the middle.
The next step is a reaming process that doesn't remove much metal but leaves a relatively smooth finish. The reamer follows the first hole and will leave a diameter that is close to the final size. Some shops follow this with a honing process, which can either be done mechanically with a traditional hone or by a hydraulic process that pumps a thick fluid abrasive compound through the hole.
There are several ways of establishing the rifling pattern and size. Two--broaching and ECM or electrochemical machining--are generally used for shorter barrels on revolvers and pistols.
The author firelapped his bore with Tubb's Final Finish bullets, which come in various levels of abrasion. By firing each series in succession, bore imperfections are smoothed out, which can improve accuracy.
For rifles, one of the first methods, which dates all the way back to the dawn of rifling, is called hook or cut rifling. A single hook-shaped cutter is pulled through the bore, and the work turns to establish the twist rate. The depth of the cut is gradually advanced by as little as a few .0001 inch at a time. It is a very slow process. Originally this was done with manpower, but a machine was developed to do the job in the 1800s.
Barrels for the 1903 rifle made at Springfield Armory were cut rifled. The high production demand caused by World War I led engineers to come up with an ingenious change to the design. By reducing the number of grooves from four to two, they essentially doubled production.
Cut rifling is still done today on a small scale, and some highpower shooters believe cut-rifled barrels are the most accurate around.
By far the most common method today is button rifling. In this process, a carbide button with a negative impression of the rifling pattern is either pulled or pushed through a reamed bore. The button is actually larger than the reamed hole, so the rifling is swaged into the steel. There is a bit of black art here because the barrel first expands and then springs back to a finished diameter that is a bit different than the original reamed hole.
A camera/bore scope setup from Gradient Lens allowed Petty to view the progress of his firelapping exercise.
This method is so widely used because the process is well-understood and also because it is fast and economical. While rifling buttons are expensive, they can produce a large number of barrels before they wear out. But the key to a smooth button-rifled bore comes in the step preceding rifling. If the bore is not reamed smooth to begin with, no amount of swaging by a button is going to make it smooth later.
In the hammer-forging process, a reamed blank is pounded with heavy hammer blows as a mandrel bearing the rifling pattern is slowly moved through the barrel. The machines that do this are huge, and the mechanical hammers hit the work with hundreds of blows per minute. The initial blank will grow significantly in length when done.
It is widely assumed that the smoother the bore is the better it will shoot. Maybe so, but barrels seem to be laws unto themselves, and it has been shown time and again that barrels produced consecutively to one another may be very different when tested for accuracy.
There are many factors that influence the quality of the finished bore. Obviously the quality and sharpness of the cutting tool is important, but so is the cutting fluid that carries away chips--not to mention the speed of rotation and also the rate at which the cutter is advanced.
At the top of the accuracy scale we can find barrels that are finished by a lead-lapping process. This can be done either by hand or machine. A lead slug is poured onto a rod in the barrel, and a relatively fine lapping compound is applied. As the lap is moved back and forth through the bore, it polishes and evens out any high spots.
Hand lapping is the most costly, but proponents say a good craftsman can feel small differences in the bore and apply more or less work to one spot as needed.
When the rifle in question came, I mounted a new Leupold VXIII Long Range scope in 8.5-25X and headed for the range. After a break-in session that involved cleaning after every round for the first five, every two rounds for another six and then a final five-shot group, I conducted the initial accuracy test from a cold, clean barrel; results are shown in the accompanying table.
While the level of accuracy I got makes the much desired minute of angle, I have come to expect Savage rifles to shoot even
better, so as I mentioned, when the shooting was done I dug out the Hawkeye bore scope and found what I deemed excessively heavy tool marks on both lands and grooves the length of the barrel. I hasten to mention that it is exceedingly rare to see a flawless mass-produced barrel, and my other Savage has a much better looking barrel, so I consider this one an anomaly.
Some years ago I had done some testing of the firelapping process and gotten very satisfying results, so that was the next step. I know of two competing products: Firelapping from NECO (neconos.com) and Final Finish by G. David Tubb (Superior ShootingSystems.com).
My previous experience was with the NECO product, so I elected to try Final Finish this time. Tubb recommends a 50-round process with 10 each of five different grades ranging from coarse to very fine. In past efforts I'd gotten good results by shooting only five of each, so that was how I started.
The procedure calls for loading using the starting load of a relatively fast powder appropriate for the caliber and bullet weight. The .30 caliber kit I ordered came with 175-grain bullets, and the fastest powder I had was IMR 3031. The Hodgdon manual shows a starting load of 38.0 grains, so that is what I used. The starting load is suggested because the abrasive coatings might increase pressures a bit, and the only goal is to push the bullet through the bore anyhow.
The toolmarks on the lands and grooves left behind by the rifling process. After firelapping, there's a bit less marking.
I made no effort to chronograph or shoot groups. I simply shot the five rounds, cleaned the bore thoroughly, and shot the next grade. As this progressed, the effort required to push a patch through the bore became noticeably less. When the first 25 were fired, the bore was inspected and photographed.
I have owned a Hawkeye bore scope for some time, and when I began this project I ordered a camera adapter from the manufacturer Gradient Lens Company (gradientlens.com). My first photographs were pretty dismal, and a call to Gradient Lens got me some good advice. Following their suggestions, my results improved but were not great, so they kindly lent me a bore scope and camera chosen specifically for my task. This included a brighter light source that gave dramatically better results.
After the first 25 rounds you could see a small improvement in the look of the bore, and that showed up on paper as well. I wanted to see what I could get after another 25, so I repeated the process per the Final Finish instructions.
Every time I do a test like this, I prove one immutable shooting truth: You are always in for a surprise of some kind. The ammunition chosen was selected because I had enough of it from the same lot to be able to control that one variable. Even then we can see some differences in velocity, which may be attributable to atmospheric change as well as smoothing of the bore.
By the end of the second 25-round process it was noticeably easier to clean the bore, and patches came out relatively clean after three or four dry ones.
Even though visual inspection of the bore showed continuing improvement, there was significant copper fouling that took quite a bit of work to remove. In the end I used the Outers Foul Out to get it truly clean, although the strong solvents such as Sweets, Barnes or Outers Super Solvent would also do an acceptable job.
While the chart shows a trend toward better accuracy, the effect seems to be greater with the heavier bullets, and in the end it is probably safest to say that now the rifle shoots equally well with both weights.
Few shooters have the benefit of a bore scope, and without it most would have thought, as I did, that accuracy wasn't fine. But I also knew there was a simple, inexpensive way to improve accuracy. Both firelapping system makers offer ammunition loaded with the treated bullets or component bullets you can load yourself for most popular calibers.
Other rifles I have firelapped before have shown more dramatic improvements but, as I said, this particular rifle delivered one m.o.a. right from the factory.Final Finish treatment brought it down to a tad under 3/4 m.o.a. Not too many mass-produced rifles do that, and I'm sure that when I work up a handload or two for it things will be better still.