The .22 Long Rifle reaches its maximum velocity in a barrel that is about 19 inches long, so Scott experimented inch by inch with barrel length and the .17 Mach2 to see where and if its length mattered.

Barrel length has an effect on muzzle velocity, but does an inch or two one way or the other make a meaningful difference? Is it possible to have a barrel so long that it reduces velocity? Is there an optimal barrel length for highest velocity? These are questions that often come up among shooters, and I recently had the chance to answer them as regards the .17 Mach2 (.17M2) rimfire cartridge.

I just completed a report on the Thompson/Center R55 semiautomatic in .17M2, which will appear in an upcoming issue of Shooting Times, and during that project I had occasion to shorten the barrel on a T/C Contender also chambered for .17M2 to the same 20-inch length as that of the T/C R55.

The purpose of the amputation was to compare muzzle velocity of the fixed-breech Contender to that of the R55 autoloader to see if there was any significant velocity loss from the self-loader. There wasn't. In fact, the R55 produced slightly higher velocity. But the project also presented an opportunity to cut the Contender barrel off in one-inch increments to see if there was an apparent optimal barrel length for velocity that I could ascribe to the little .17-caliber rimfire.

Published experiments on how barrel length affects muzzle velocity have been done with the .22 Long Rifle, and from those experiments it has been concluded that generally any barrel length greater than 18 inches is actually causing the .22 Long Rifle bullet to slow down. The precise optimal barrel length for a .22 will vary from one load to another and one gun to the next because of different powder charges in the loads and tolerances in the bore dimensions. Regardless, the reason for the bullet slow down at that short a barrel length is because the expansion ratio (the sum of the volume of the bore and powder chamber divided by the volume of the powder chamber) for the .22 Long Rifle is so high. In other words, it has a very small powder chamber relative to the bore.

As powder burns it increases in volume about 1000 times, which increases pressure if contained as within a chamber. That pressure starts the bullet down the bore against the engraving forces, bullet-on-bore friction and the pressure of the air in the bore in front of the bullet. As the bullet travels down the bore, the volume of the space behind the bullet is increasing such that after reaching a certain point, gas pressure no longer increases.

After an inch of barrel was cut off, the muzzle was crowned and 20 shots were chronographed and averaged to determine what length was optimal for highest velocity.

Eventually, the gas pressure and bullet friction reach a point of equilibrium, followed by a transition to the effects of bore friction being greater than gas pressure. If the bullet is still in the bore after that transition, it slows down. While it happens at around 18 inches in a .22 Long Rifle, it would take a barrel several feet long in a cartridge such as the .308 Winchester because it has a much lower expansion ratio. The closer you get to optimal barrel length for maximum velocity, the less significant each increase in velocity becomes, which is why we get along fine with sporter barrel lengths in centerfire rifles.