When I started shooting airguns, 1000fps was an unheard of velocity in airguns. Today, every maker seems to offer a variety of guns advertised as 1000fps guns, and one maker brags that his rifle produces 1200 fps. Has technology improved that much?
No. What’s happened is that makers are testing guns with very light pellets, and in some cases putting in springs that are far too strong for the gun’s design. resulting in poor accuracy and early failure- all in pursuit of the fabulous 1000fps barrier. But these number are very misleading- and making a pellet go 1000fps or faster is not really very desirable. Here’s why.
Velocity alone tells you nothing; a felt cleaning pellet will easily exceed 1000fps in most rifles, but it won’t go very far, and it won’t do much damage compared with a lead pellet. What really characterizes the amount of power available in an airgun is the muzzle energy, measured in foot-pounds. To compute muzzle energy, you need to know the weight of a pellet, in grains, and the velocity of the pellet. This can be measured with an inexpensive chronograph like the CHRONY- an invaluable tool for any serious airgunner, by the way.
(These start at $89.95- I’ve owned mine for at least ten years. You can find them at most airgun suppliers, or buy them direct from the company.) Once you know the velocy and the weight of a pellet, the formula for energy is:
E = mÂ·v2/450437
Let’s consider a gun with an advertised 1000fps muzzle velocity when tested with light 6.5 grain pellets like the Beeman Hobby pellet:
6.5 * 1000 * 1000 / 450437 = 14.43 ft-lbs.
That’s not bad, but consider that my Theoben acheives 980fps with 7.9gr pellets- that’s almost 17 foot pounds- with a slower pellet.You can see right off that velocity doesn’t really tell you what an air rifle is capable of- muzzle energy is a much better measure. And that slower, heavier pellet has a number of advantages beyond having more energy to begin with.
As a pellet approaches the speed of sound- 1000fps- drag shoots up exponentially. A lot more energy pushes the pellet only a little faster. It’s very inefficient.
Second, as the pellet rapidly deccelerates from supersonic to subsonic speed, it goes through the transonic region- a region of great aerodynamic instability. It’s better in general to start out subsonic and stay there, or, if you have enough energy available, start supersonic and stay there thoughout the flight.
Airguns don’t have all that much energy, though; a .22 Long Rifle cartridge has over 90 foot-pounds of energy available- quite a bit more than all but the most powerful large-caliber airguns. Yet target .22 cartidges are designed to propel the bullet at subsonic velocities.
Generally, then, when selecting pellets for an airgun, it’s a good idea to start with pellets that stay subsonic. Target guns with 5-7 ft-lbs of muzzle energy work well with light (6-7gr) target wadcutters. Guns over 12 ft-lbs should use heavier pellets, And guns producing close to 20 ft-lbs in .177 should use the very heavy pellets, like Crosman Premier 10.5gr pellets and Beeman Crow Magnums.
Another advantage of heavier pellets is that they have a higher sectional density for a given caliber. That means that the ratio of mass to diameter is higher- and that means proportionally less drag for a given level of energy. Another advantage is more resistence to wind- and you can start to see why some field target shooters choose heavier pellets even though they’re less efficient than some lighter pellets in a given gun.