I calculated the expansion ratio for a .22 Long Rifle in an 18-inch barrel and came up with 39. For the .308 Winchester, an 18-inch barrel results in an expansion ratio of about 7. To get an expansion ratio of about 39 with the .308 Winchester takes a barrel around nine feet long. That does not mean a .308 Winchester obtains its highest velocity in a nine-foot long barrel, nor is an expansion ratio of 39 a magic number. There are other factors that influence velocity, including the difference in engraving force and coefficient of friction between the outside lubricated lead .22 bullet and the copper jacketed .308 bullet. There is also the difference in surface area between the base of a .308-inch bullet and a .224-inch bullet to consider.

To keep the muzzle the same distance from the chronograph skyscreens as the barrel was shortened, it was necessary to slide the Caldwell Tack Driver shooting bags forward to keep the muzzle in line with the indicator on the shooting bench.

So all this brings me back to the Contender project and cutting off its barrel in one-inch increments and how the velocity changed as a result of it. Because the .17M2 has such a small powder chamber (it measured .0176 in3), I assumed optimal length would also be about 18 inches, as with the .22 Long Rifle, so I started with a 23-inch factory barrel. Using an Oehler Model 35P chronograph set up with four-foot screen spacing at 15 feet from the muzzle, my procedure was to chronograph and record five shots, then clamp the barrel in a Wheeler Engineering barrel vise, cut off an inch using a hacksaw and record five new shots.

With each inch, velocity declined until I got to 20 inches, then velocity appeared to climb again. By the time I had the barrel cut to 17 inches, velocity appeared to be going back down. Physically, that little increase in the velocity can't occur, and indeed when I checked the data using Tioga Engineering's Baltec1 program, it showed that the blip in velocity was not statistically significant and might not have actually occurred.

Blip or not, I decided to repeat the test using a special 27-inch barrel T/C made for the occasion and to fire 20 shots per inch of barrel to try and lower the standard deviation. I also made it a point to control my variables a lot better by crowning the muzzle between cuts using a brass round-head screw and valve-grinding compound in a cordless drill.

This time I also cleaned the bore thoroughly between groups of shots using a Hoppe's BoreSnake, fired one fouling shot before recording velocities for each group of shots, and used an indicator on the shooting bench top to make sure that regardless of barrel length, the muzzle was always at the same distance from the "start" chronograph screen. This time, muzzle velocity appeared to increase steadily as the barrel was shortened until a barrel length of 23 inches was reached. There was no "blip" in velocity like I had with the previous barrel, and velocity appeared stable between 16.5 and 23 inches of barrel length. A line graph of those results is shown nearby.