Because of the .17M2's small powder chamber, I assumed it, like the .22 Long Rifle, had a high expansion ratio and expected similarly to find a barrel length near 18 inches where velocity would be highest. To check expansion ratio, I measured the volume of the little .17's powder chamber by pulling a bullet, dumping the powder, filing a small groove along the length of the bullet, and then weighing the two in grams on a PACT electronic scale. Next, I filled the case with water, reseated the bullet allowing the water to be displaced through the groove I had filed, and weighed the assembled cartridge. The difference between the empty and full case is the weight of the water in grams, which, through the magic of the metric system, was also the volume of the powder area in cubic centimeters, which I converted to cubic inches.

The average velocity for 20 rounds with each inch of barrel was plotted and the data was "smoothed." Scott's results agreed with what Hornady found when developing the .17M2 in that sporter-length barrels produce essentially the same velocity, though 19 to 20 inches seems to produce the highest velocity.

Bore volume was obtained by multiplying the effective length of the barrel (distance from the boltface to the muzzle, plus the seating depth of the bullet, minus the case length) by IIr2. That value was taken at 98.5 percent to account for volume displaced by rifling. Measured as such, the .17M2 fired in an 18-inch barrel has an expansion ratio of about 25.

With the special 27-inch barrel T/C made, expansion ratio is around 38, so I reasoned 27 inches would be about optimum for highest velocity with the .17M2 and expected velocity to decline with each inch of barrel less. Because my experiment indicated otherwise, I contacted Dave Emary of Hornady to see if he had done any similar experiments when initially developing the .17M2.

"We did the same thing very early on with a minimum spec test barrel. We found that with barrel lengths over 21 to 22 inches, we started to lose velocity. Barrel lengths from 16 to 21 inches produced virtually the same velocity," Emary said.

You can clearly see on the line graph that velocity for the .17M2 is reduced using a barrel more than 23 inches long. From previous discussions with ballistics experts William C. Davis Jr. and Charles R. Fagg of Tioga Engineering, I knew there could be more information in the raw data than was readily apparent. I consulted with them, and Davis was kind enough to use various data smoothing techniques to find a smooth curve that best fit the raw data. Smooth curves filter out normal fluctuations in data allowing you to see through to the changes occurring in the sample. From the nine different curves Davis tried, a parabola was chosen as the best fit. The smoothed data is shown as the red trend line.

With the data smoothed, it showed that maximum velocity for the .17M2 is obtained with a 19- to 20-inch barrel and, as Hornady found in the development of the cartridge, that velocity is essentially the same for all sporter barrel lengths. What this means to a .17M2 shooter, then, is that you can opt for a rifle with a shorter barrel and not sacrifice anything in the way of velocity, and if maximum velocity is what you're after, you should realize it with a barrel 19 to 20 inches long.