Catapult guns and velocity
by Tom Gaylord
Writing as B.B. Pelletier
Sharpshooter pistol uses rubber bands to launch a .12 caliber lead ball. Other catapult guns were as large as .43 caliber!
This report covers:
- You know catapults guns
- More power doesn’t mean higher velocity
- Why a limit?
- What is the limit?
- Crossbows may be faster — but…
- What about stonebows?
You know catapults guns
Over the years I have written several reports about catapult guns . The Sharpshooter shown above and the Bullseye pistol that proceeded it used rubber bands to launch their shot. But the Johnson Indoor Target gun used surgical tubing. And a don’t really know for sure what the .43 caliber Hodges gun of the 1840s used but I suspect it was natural rubber bands. The point is, catapult guns have used many different power sources.
Johnson Indoor Trainer uses surgical rubber tubing.
More power doesn’t mean higher velocity
Most people equate the strength of the elastic bands with velocity, but with catapult guns it doesn’t work quite that way. It is possible to increase velocity by adding additional bands or stronger bands to a point, but once that point has been reached, no more velocity is possible. There is a close correlation between catapult guns and spring-piston guns in this respect. though there are also a few significant differences. For today, let’s stick with catapult guns.
I discovered during testing of the Sharpshooter and Bullseye pistols, that by adding additional rubber bands I could boost the velocity from around 100 f.p.s. all the way up to about 170 f.p.s. Then it stopped rising and I couldn’t fit any more rubber bands on the guns. If I worked at it, there is probably 30 more f.p.s. to be had, but that will be the limit.
Why a limit?
This is the crux of the question, and it also applies to the metal springs, found in conventional pellet guns. The larger a spring becomes, the greater its mass. The greater the mass, the slower that spring operates. It will push more weight, but it will do it slower. That defines the velocity limit of a conventional spring.
A stronger but slower spring will push more weight at a given velocity. In some catapult guns where the shot size is limited by te gun’s design, you have to find the optimum spring size for your projectile. In other catapult guns where there is greater latitude in the projectile size and shape, a larger spring means greater weight can be pushed. A gun like the Hodges will benefit from using the biggest ball made from the heaviest substance. That would be a .43 caliber ball made from pure lead.
The Hodges catapult gun from the middle 19th century shoots a heavy lead ball.
Other catapult guns may allow the use of different-shaped projectiles. This is where the speargun comes from. And some guns may be convertible to shoot either spears/arrows or round balls. Match the projectile to the application. But with all of them the size of the spring — be it a rubber band or a thick natural rubber cord — determines the velocity. More power does not mean more speed.
What is the limit?
For many years I have written that the Hodges was capable of taking boar-sized wild game, based on an estimated velocity of around 350 f.p.s. Well, where does that number come from? It came from a remark made by Larry Hannusch, when I was talking to him at an airgun show about his Hodges gun. I asked him if he ever shot his gun and he said he had. But he never really chronographed any shots from it. He had loaded both sides of the launcher with rubber tubing, one band at a time, until the steel arms of the launcher appeared to be stressed. That was where he stopped. When I pressed for a velocity number he rolled his eyes and said, “I don’t know. Maybe around 350 f.p.s.?” It was a best guess.
The point is, no velocity testing of a Hodges guns has ever been done — as far as I know. But there has been testing of the catapult concept by itself. In his book, Man-Powered Weapons and Ammunition (copyright 2005), by Skyhorse Publishing, New York, Richard Middleton did a test to find the limits of several catapults. While doing this I believe he stumbled upon the fundamental fact that all catapults have a limit.
That limit seems to be around 250 f.p.s. It seems that nothing can be done to increase that limit. If more elastic bands are added the gun will launch heavier projectiles just as fast, but nothing will make them go faster. Maybe this should be called the elastic limit or barrier?
Middleton did not actually conduct his tests to find this limit I am describing; but all of his tests of catapults seem to end just shy of 250 f.p.s., save one. A condom firing a chickpea was able to reach 270 f.p.s. — or so it was claimed. While Middleton never states that 250 f.p.s. is the practical limit of a catapult, I am suggesting that it is, based on all of his results.
Just as additional mass in a coiled steel mainspring actually slows the spring down, so does adding more elastic material to a rubber band or surgical tube. This mass/accelleration ratio is what defines the limit of a catapult.
Crossbows may be faster — but…
I say that crossbows may be faster than other types of catapult guns, but their upper limit seems to be around 425-450 f.p.s. That’s more velocity than ball-shooting catapults are getting, which makes crossbows more efficient. And obviously I’m talking about crossbows with conventional limbs and a string — not airbows.
What about stonebows?
Some of you are aware that there are such things as stonebows. They are crossbows designed to shoot stones or balls. Middleton did test several stonebows, though his tests were by no means comprehensive. The max velocity he got hovered at just over 200 f.p.s. So the stonebow falls well in line with the elastic limit.
I conclude from both Middleton’s writings and my own limited tests that 250 f.p.s. or thereabouts is probably the limit that a spring-powered gun can achieve. If I am right, we now know something important about airgun design. If I am wrong, whoever disproves me will advance our understanding and knowledge.
The bottom line is this — catapult guns are slow. And they will always be slow because of the limits of the springs that power them.