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Measuring performance

chrony Airgunners like to talk about velocity- no one’s magnum gun is ever shooting less than 950fps- and power- this is a 20 foot-pound gun, right? Of course. But the only wayto actually know for sure is to measure the velocity that a gun is producing. And the only way to do that is with a calibrated chronograph.

Back in the 1960s and 70s, airgunners would actually try to compare velocity by comparing how deep a pellet penetrated a piece of “ballistic putty”- i.e., filled clay. This will show the difference between a 5 foot-pound gun and a 10 foot-pound gun, but it’s pretty useless for determining whether that supertune you just gave your gun increased- or decreased- the power.

Luckily in the 1980s the Shooting Chrony came along- the first chronograph that was so cheap there was no reason not to buy one. The basic model is still only $89.95, and it’s very accurate. And Shooting Chrony is a company who understand shooters; they’ll give you a very good trade in on a new unit if you happen to put a bullet through yours! Pellets generally don’t penetrate steel, but I personally put a pellet into my shooting partners’ Chrony, destroying a sensor. Luckily the spare parts were readily availible and reasonably priced. They also offer a clear plexigas shield that will stop or deflect a pellet, arrow or paintball, but allow you to read the display. For a few bucks more you can get units with more memory, more functions, and computer interfaceability- a nice feature if you do a lot of tuning or testing.

Once you have a good measurement of velocity, the next step is to compute the actual muzzle energy. The formula is simple. Where v=velocity in feet per second, and m=mass of the pellet in grains, muzzle energy in foot-pounds is equal to:

(v2 * m)/450240

That’s velocity squared times mass, divided by 450240. (That odd number results from multiplying the number of grains in a pound times the gravitational constant)

Once you have the actual muzzle energy you can compare different pellets and find out what pellets actually peform best in your gun. But prepare to be suprised, and even disappointed; a lot of guns don’t produce anywhere near the velocity or the energy that their ads promise. I was surprised to learn that my HW77 produced only about 10.5 foot pounds of muzzle energy. It was still a very accurate gun that won matches, but I was absolutely sure until I tested it that it was a 12 foot-pound gun. Similarly my 20 foot-pound Theoben Sirocco turned out to be a 15 foot pound gun at 900fps- and only a 12 foot-pound gun with heavier pellets. It puts Crosman Premiers through a 1/4″ hole at 60 yards, but I still felt a little let down.

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