Going back to our example, let's say our 2 inch barrel is 4 feet long (48 inches), and behind our barrel and pilot valve, we have a 3 inch chamber that's 2 feet long (24 inches).īarrel Volume = 1 inch^2 x 3.14 x 48 inches = 150.72 cu.inĬhamber Volume = 1.5 inch^2 x 3.14 x 24 inches = 169.56 cu.in All we need is the volumes of our air chamber and barrel to calculate the pressure drop.

We need to account for this drop in pressure, but fortunately we won't need any calculus-level math for this. 314 pounds is just our INITIAL applied force. So the force on our projectile is not constant throughout. This goes to show why we should use pressure rated material for our spud guns, seeing how the force exerted on the projectile alone is already the weight of a very large man/woman, at 100 psi we're already rivaling the draw weight of a Roman ballista.Īt this stage you might be inclined to simply multiply your pounds of force by the length of your barrel to get your foot-pounds of energy, but we have to remember that as our air pressure expands into and down our barrel, the pressure goes down. Now we multiply the pressure you'll be using by this base area, which assuming in our example was 100 psi, comes out to:ģ.14 sq.in x 100 psi = 314 pounds (force) You have a 2 inch barrel, so with a nicely snug flat bottom* projectile, the base area would be: Please note this only works for pneumatics. If you can roughly account for any friction or flow efficiencies, the result is reasonably accurate (I have not confirm this myself seeing that I don't have a chronograph, but I have done the math for a few spudguns that have been chronographed by their builders, and using just their dimensions I come pretty close to their results).įirst you need to find the area of the base your projectile, usually its just finding the area of a circle (unless you're using a square barrel ). I use pressure setting and gun dimensions (volume, length.etc) to calculate my muzzle velocity and energy. A bullet does about the same thing, except it has a lot more energy and it also causes damage through hydrostatic shock to your vitals. The tiny cross-section of a sharp crossbow bolt insures it transfer the maximum amount of force onto a small area to penetrate its target, and hopefully cutting a jugular or vital organ.

But take that same bucket, put a spike on the bottom, and it would do about the same damage to your head as a bolt fired from a crossbow. Yes, in normal circumstance a bucket dropped on your head would hurt, but not lethal. A 3.5 gallon bucket of water, dropped from two feet, also hits with 60 foot pounds of energy. A crossbow bolt has about 60 foot pounds of energy behind it when it hits its target. It depends on what you're projectile was meant to do.

Muzzle energy calculator