The Relationship Between Barrel Length, Velocity And Accuracy
Apparently, many shooters are very concerned with velocity loss in shorter-barreled rifles. How many times have you heard or read that a .300 Magnum with a 22″ barrel is only a .30-06, or that a .257 Weatherby Magnum has to have a 26″ barrel to perform to its potential? There’s also the oft-quoted guess that velocity loss with any modern rifle round will be 25 to 30 feet per second per inch of barrel, and the common belief that faster-burning powders lose less velocity in shorter barrels.
Even with the widespread use of personal chronographs, a lot of such stuff gets tossed around these days, perhaps because few handloaders own a 22″-barreled .300 Magnum or are willing to take a hacksaw to their favorite deer rifle. Some people, however, perform actual experiments with different barrels lengths. As usual, reality is less predictable and more interesting.
My first experiment took place over 20 years ago, with a pre-threaded Shilen 7×57 barrel on a VZ-24 Mauser action. I asked the gunsmith to leave the barrel at its original 24″ length, and then fitted the barreled action into an aftermarket synthetic stock. After trying a few handloads, I hacksawed 3″ off the barrel and re-crowned the muzzle with a Brownells hand tool, then retested the same loads over the same chronograph.
The loads with the widest spread in velocity loss were the 140-grain Nosler Solid Base with 51 grains of IMR4350, and the 160-grain Nosler Partition with 50 grains of H4831. The difference in amount per inch was considerable: 15 fps for the 140 grain versus 41 fps for the 160, neither very close to the oft-quoted 25 to 30 fps, though the average velocity loss for the two loads was 28 fps.
A third load, using the 130-grain Speer Hot-Cor and IMR4064, lost 29 fps per inch, right where it theoretically should be—except for the fact that 4064 was by far the fastest powder. There’s an excellent reason it lost just as much velocity, on average, as the other two loads: Almost all the smokeless powder in modern rifle cartridges burns in the first few inches of the barrel. (This may startle some people—but that’s another subject, perhaps deserving another essay all by itself.)
The next experiment occurred when a well-known firearms company sent a .300 Winchester Magnum for testing. The name of the company won’t be revealed, since management has changed in the years since. The 24″ proved to have some rifling totally missing close behind the muzzle, which made it shoot more like a smoothbore slug gun than a rifle.
I contacted the company’s public relations guy and asked permission to perform an experiment, and he pretty much shrugged. The barrel was cut to 22″ and re-crowned, to see if the loss of 2″ of barrel does turn a .300 Magnum into a .30-06. Velocity loss per inch varied between 51 and 79 fps with various loads, but even those lower muzzle velocities exceeded any 22″ barreled .30-06 I’ve ever chronographed, and there have a been a bunch.
The results of both experiments appear in the accompanying chart. Cautious readers will notice the powder charges are somewhat higher than those listed in today’s data. There are two reasons: Most loading data was a little higher back then—and I was younger and dumber, occasionally even trying to turn a .30-06 into a .300 Magnum.
Since then, similar experiments have been performed with several barrels, but none have proved significantly different than those first two trials. Velocity often varies more than 25 to 30 fps per inch of barrel, and there isn’t any real pattern to which cartridges lose more velocity—though “magnum” rounds will lose a little more, on average.
By John Barsness
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