November 2000

Barrel Comparison Test: To Flute Or Not To Flute

Does fluting high-performance rifle barrels make a difference in accuracy or heat dissipation? We conducted a before-and-after test on a .22-250 match-grade barrel to find out.

[IMGCAP(1)] We have always shied away from fluting barrels. As an old machinist told us many moons ago, the more you machine a piece of steel, the more chance you’ll generate problems. For instance, one problem barrel fluting can cause arises from dulling tools. The cutter gradually dulls as the flutes are milled, and they tend to generate a hard surface case in the bottom of the flutes. This is not a problem if these areas of hardness are the same. But the process tends to produce a variable hardness in the flute-groove bottom. As the flutes are cut, the cutter is gradually dulling, and the first groove is not as hard as the bottom of the last groove. This can cause stress in the barrel, and inaccuracy results.

There are other problems which make us wonder if fluting high-performance rifle barrels makes a difference in accuracy or heat dissipation, so we conducted a before-and-after test to find out. It was not our goal to find out how a bad fluting job affects accuracy, so all grooves were cut with a final pass using a new milling cutter. Also, as reloaders and shooters, you know that accuracy is a game of uniformity, so we tried to hold the maximum uniformity during the fluting process.

Why Fluting?
Despite the popularity of fluting, there are significant downsides to the process. Ask a barrel maker about fluting, and he will cringe and make very ugly faces. He knows that the fluting process makes the base of his bad-barrel “bell curve” much wider, thus increasing his scrap rate. In fact some of the top barrel makers like Kreiger and Obermeyer like to finish all external dimensions on fluted barrels before the rifling is cut—just to reduce problems. In a conversation with technicians from Lothar Walther, the German barrel makers, they frowned and asked, “Why do you want fluting?” If cooling is a problem, they said, use an external shell of aluminum (finned) shrink-fit over the barrel. This is similar to the barrel-cooling shrouds found on modern antiaircraft guns. They recommend this solution even for fully automatic fire in barrel applications where temperatures exceed 300 to 400 degrees.

As you can see, we were skeptical about fluting’s value, so we endeavored to test a barrel before and after fluting for accuracy and before and after to assess its cooling rate. The gun selected was a VZ-24 Czech 98 Mauser made prior to WW2. It was drilled and tapped for a scope mount (Redfield) and a new bolt handle installed. We installed a 26-inch Lothar Walther barrel, and cut the .22-250 chamber with a reamer from JGS. The Walther barrel was a 1-in-14 twist with a non-stainless steel alloy of chrome vanadium. Its steel was produced by Krupp in Essen, Germany, to rigid proprietary specifications for Walther. This barrel was turned to a European-type profile with a larger O.D. (0.875 inch) near the muzzle prior to its installation. This was bedded into a MPI fiberglass stock and left with a fully floating barrel. We did not give a high polish either prior to fluting or after fluting to ensure that the same surface roughness existed for both test series. Prior to barrel installation, we lapped the barrel so that the new barrel was closer to being broken in, giving more reliable data. We topped the gun with a 6-18x Bushnell scope with an adjustable objective lens.

Our temperature testing was done with a Exergen Corp. optical infrared thermometer. This D500F unit employs technology used in thermal tank sights; it is used industrially to measure temperatures of industrial equipment. To ensure we read identical areas before and after, we painted test spots on the barrel for the unit to read. Bright steel has a low infrared emissivity that interferes with the temperature reading, and the painted spots eliminate this problem. Our temperature measurements were taken in 2-minute intervals in four spots on the barrel.

The temperature readings were taken on a barrel that had been placed in a 250-degree oven and allowed to set (soak) for 20 minutes. The barreled action was tested as a unit and was supported on 20-gauge wires to eliminate heat loss seeping into an object it came in contact with. The cooling tests were done in still air to ensure consistency.

We conducted the test by heating the Walther barrel before modification and recording its rate of cooling. We also shot test groups to establish its “before” accuracy standard. Then we fluted the barrel and reran the tests, comparing the same barrel, unfluted and fluted, in identical conditions. Here’s what we found in the test:

The Results
The accompanying graph

(click here to view)

shows the meat and potatoes of the cooling issue—a supposed advantage of fluted barrels. We found that the most cooling advantage occurred in the first 18 minutes of the test. After that, the temperatures fell nearly together. Rates of cooling came very close to 0.75 degree per minute both unfluted and fluted. Since a 20-shot string in this barrel bumped temperatures only to the 170-degree range, and we tested at 225 degrees, we’re confident that standard-use barrels would not heat beyond this ceiling, and if cooling were markedly different at 300 to 400 degrees, most readers would never need it anyway.

Another factor, rigidity, can improve during the fluting process, and this additional stiffness can improve accuracy. So we also compared the barrel before and after to determine if fluting had helped in this department.

Accuracy testing was done by firing composite groups with 20-shot strings, using good loads that we knew to be uniform in accuracy across several similar rifles. All groups were fired without cleaning during the shot strings. We allowed 45 seconds to 60 seconds between shots and selected consistent range conditions in the early morning when winds are at their lowest and mirage is negligible.

Our test results showed some accuracy improvements after fluting. Our testing with a 55-grain Remington hollowpoint R22502 bullet showed a reduction in group size of 18 percent with a 3,594 fps average velocity and a mean absolute deviation of 29.4 fps (before and after). The next load tested came from Ken Waters in his pet load book, a 53-grain Sierra hollowpoint match bullet on top of 35.25 grains of IMR 4895 using a Winchester large rifle primer. These groups were reduced 8 percent after fluting. The last load was a benchrest favorite, 32.0 grains of IMR 3031 powder under a 55-grain hollowpoint Remington slug. The primer was a CCI large rifle primer and overall length was 2.400 inches.

This loading gave a 17 percent reduction in group size after fluting. Speeds with this load were 3,421 fps average with a mean absolute deviation of 14.1 fps. Again, we would like to reiterate that the loads were selected for their past uniformity, and no load fine tuning was done. All were loaded on a single-stage RCBS press, and primers were seated with a Lee priming tool. All brass was Remington, trimmed and full-length resized.

Thus, our average group-size reduction after fluting was 15.7 percent across the board.

Gun Tests Recommends
If you want to flute a barrel so that it cools faster, don’t spend the dollars. Our tests showed most cooling took place during the first 18 minutes, and this difference was very small.

If you want to flute a barrel so that it’s lighter, you might consider instead simply choosing a different barrel profile. Our fluting process lightened the rifle by roughly 6 ounces, which on most guns would not justify the cost. However, if you’re building an ultralight rifle on which you’re shaving every ounce, then fluting might be worth a look.

Surprisingly, if you want to flute a rifle to improve how it shoots, the process may be worth the money—assuming it’s done properly. As noted above, our average group-size reduction after fluting was 15.7 percent, with one load showing an 18-percent shrinkage in group size. Still, we make this recommendation with a caveat, since the accuracy improvement shown in this test could still be a function of break in.