September 23, 2010
Think you need a new barrel? Here are some ways to evaluate yours before taking the plunge.
By David Tubb
A barrel is continually developing, either increasing or decreasing in accuracy. How long it shoots its best, or what you deem to be its best, is a good determinant of how good the barrel is.
Judging quality, in my opinion, is properly done by shooting groups at long range (600 or 1,000 yards, depending on the load). I test in 10-round groups, and I need a lot of 10-round groups to get a reliable average.
Competitive use can tell you what you need to know about how well your barrel is shooting, but if everything is gauged off of groups fired during matches, then there's a lot of impetus to change barrels frequently, or at least to change barrels prior to seeing a poor group (poor score) from a worn barrel.
A barrel will normally shoot well longer than we anticipate, but it will go bad eventually.
I have heard people say that their barrel shot out in the middle of a string. Well, I have never seen a barrel that went out in the middle of a string. They go gradually. One of the best indicators is how much velocity has fallen off.
For instance, I used the same .308 Winchester barrel two years in a row at Camp Perry. I had shot a lot of rounds through it, always the same load. The telltale sign was when my notes indicated that my 1,000-yard elevation was more than two minutes of angle different with the same propellant charge.
It was still shooting well, but two feet on the target corresponds to a substantial velocity loss. I hadn't increased the load to compensate for the velocity loss, which is something I do now; I kept shooting it because it shot well.
If I kept checking the chronograph every so many rounds (each 1,000 or so on a .308) and kept loading it back up to keep velocities up I probably wouldn't have seen that difference.
Barrel wear in a centerfire rifle comes almost exclusively from throat erosion (cracks and roughness in the first several inches ahead of the chamber, caused by heat, flame and pressure). If a shooter wants to periodically increase the propellant charge to compensate for velocity loss, then barrel wear can be measured by throat erosion (as well as by keeping track of the powder increases necessary to retain original new-barrel velocities).
As a throat wears, it moves forward. If you're seating a bullet to touch the lands or rifling at one cartridge overall length and then find, after so many rounds, that you're seating the bullet to touch the lands at another overall cartridge length, the difference between those measurements is throat erosion.
There are no objective benchmarks with respect to velocity loss or throat wear that determine when a barrel is done, and there are differing opinions, but I wouldn't want to see more than .200 inch difference in seating depths or more than a couple grains of powder before I'd pull that barrel.
In my Tubb 2000 rifles I can easily change barrels and can test several barrels to learn what each will deliver. After spending time with a barrel, I feel like I know it, and that's one reason I don't want to take a freshly installed barrel to Camp Perry. I want to have seen it in action and have worked through all my zeros and seen how it responds to different loads.
Through the year I have been running experiments and noting behaviors and zeros, and the majority of that information, or at least the most reliable information, has come from barrels that are from 25 to 75 percent maturated. Using this system, I could effectively shoot the same barrel more than once at Camp Perry. That, I think, is an advantage.
Here's some hard-learned advice on twist rate selection. If there is any question, and an option, go the next step faster in twist rate.
I was one of the first contending shooters (early 1990s) using .243 Winchester. We were using the then-new Sierra 107-grain bullet, and trying to determine what was necessary to stabilize it. Consensus seemed to indicate that a 1:9 twist was adequate, and it was, until we got to Camp Perry.
My home is at about 2,500 feet above sea level, and I experience typical western summers, meaning it gets hot, but at the 600 feet elevation in Ohio, the 1:9 twist cost me a title that year.
I'm now using a 1:7.5 twist to drive 6mms. Using bullets suitable for NRA Highpower, there will be no ill effects from going one turn faster than you think you might need.
It has been a standard for a good while now to use a 26-inch barrel on an across-the-course rifle and add a couple inches, or more, to that for a long-range rifle.
I once tried a 6.5-.284 with a 29-inch barrel and eight-inch sight extension tube, and it produced such a vibration node that it took about a second and a half for the scope to clear up--it was like hitting a tuning fork. I knew that couldn't be good. I cut an inch off the barrel and an inch off the tube and the vibration went away completely.
You can get a tube too long, and a barrel too long. From my experiments, a six-inch tube works well with a .308, or smaller, caliber, and I think 28 inches is as long as a barrel needs to be.
Barrel contour has been another area of evolution for me. The contour I'm using now is quite a lot smaller than what I used for the majority of my career. As with other things, I tended to stay with what had been working for me, and that was originally determined by what most others were using with success. I learned that just because something works doesn't mean that there's not something better.
The barrel profile I'm currently using is essentially the smallest diameter barrel that doesn't shoot any worse than the heaviest barrel. It is about one pound lighter than the old straight-taper profile I used to use. I went to the lighter barrel to increase my options in rifle fitting. By making the barrel lighter, I could add or remove weight to the fore-end rail, and relocate it, to enhance my position-shooting preferences.
The contour starts at 1.200 inches (like always), then steps to 0.950 and tapers to 0.875 at the muzzle at 25 inches (not withstanding the 0.75-inch parallel section for front sight mounting). I am convinced that a barrel doesn't have to be one bit bigger than this to shoot well.