ROD AND GUN Determining Bullet Velocity By Home-Made Chronograph

Press, Volume CVI, Issue 31278, 26 January 1967, Page 13

ROD AND GUN Determining Bullet Velocity By Home-Made Chronograph

(Specially written for “The Press” by

JAMES SIERS)

Speed and power were valued by early man as an important feature on the battlefield and if one reads Homer, it seems he was as much prone to exaggerate the prowess of his heroes as modern riflemen are inclined to add extra feet per second to the velocity of their latest rifle.

I know of no chronograph service in the country where the issue can be settled, but within the last two years, two chronographs have been built here and are already beginning to settle a lot of arguments.

When I received the components for a .17 calibre rifle, the first reaction was to go and get some figures, particularly as the ballistics worked out by the gunsmith were based on American canister powders unavailable here.

He made some test loads with Norma 204 powder and found that 32 grains was extremely accurate and guessed the velocity to be 4200 feet per second, a speed that was all I expected to get in this calibre.

So, as soon as a number of test loads were made up, I telephoned Graham Henry’s home and a date was set for the trial. What impressed me about the chronograph made by Graham’s brother, Lyall, was the specifications. It was accurate to a hundredthousandth of a second. Also, once the bullet passed between the two screens, breaking the circuit, the speed was fixed on the dial and stayed fixed, until the dial was reset. There is no fear that the reading may not be accurate because of atmospheric conditions or because the shooter may not be quick enough to read Hie dial before the needle begins to fall-off. Surprising Result

We set up the two screens at a measured distance, switched on the batteries, threw a test-switch to see all circuits were linked, and Graham lined up his .222 for the first test Surprisingly, with a 5-shot average, he found that his 55-grain bullet was zipping along at just over 3300 feet per second, which is faster than the factory-load. Next came the .17 and with a five-shot average, with 32 grains of Norma 204 powder, the average was 3984 feet per second, much slower than the anticipated 4200 feet per

second. Next came a string of four shots, with 32} grains of Norma 204, and the average went up to 4065 feet per second, a good improvement, but bought at the expense of extra pressure. Slight cratering appeared on the primer. I have loaded an additional number of test loads with 33 grains of Norma 204 and this should take the bullet to over 4100 feet per second, a figure I will be content with.

Graham and Lyall have used their chronograph on a number of calibres, and looking up the list, I was surprised to see that on the average the factory loads, which inevitably quote maximum speed, were generally down by up to 200 feet per second.

Lyall, who is a television technician, built the chronograph. "It’s really a small transistorised computer,” he says. “It has 35 transistors and 20 diodes, and this makes it possible to have it so small. If it was not for a bank of batteries, it could be smaller still.”

The principle of any chronograph is to measure the speed of a bullet between two given points, and by computing it, to declare a muzzle velocity. Very Accurate The machine used by Graham and Lyall is incredibly accurate, because it records the time between the two screens, being broken up to a one hundred thousandths of a second, a mind-boggling speed. Fooling with reloading is a serious business. The hardness of the bullet’s jacket is sufficient to increase the pressure by up to 7000 pounds in the breech, so that if you happen to be working with a modern magnum load, which already realises up to 55,000 pounds per square inch in the breech, an additional 7000 pounds can result in either the destruction of your rifle, or your head, or both. The point is that if you change to a new projectile, drop your load to a safe minimum and then check for pressure. So, unless you’ve got access to a chronograph, the speed of your bullet is academic and the best advice is: load only what you know is safe, accurate and effective loads. If it does the job, the speed is of secondary importance. The photograph shows

Graham Henry (with beard and rifle) checking the dials of the chronograph, while his brother,

Lyall, replaces a screen. Jack Hunter, whose 6.5-06 is being chronograph,ed, is on the left.