Since this community has been so helpful, I wanted to give back what little I could. This might be obvious to all of you but it wasn't to me so I figure I'd post it for people to see. I'd love to claim credit for the math but I had one of my friends help me with it. I'm just posting it as he explained it to me.

Let's say you are using a reservoir air tank to fill up your chamber for each shot and you want to know how many shots you can get. The math to solve this at a basic level is called the "Ideal Gas Law" and can be found here. It looks like this:

pV = nRT

p = pressure

V = volume

n = "moles" usually

R = gas constant

T = absolute temperature

This is a nasty bit of work normally but we can ignore most of it for our purposes. Since we are going air to air, the n is ignored. The gas constant can be ignored as well. For the sake of keeping this simple, we will assume the temperature while filling doesn't change - this is the biggest assumption and might or might not have a big impact on the results depending on the differences in pressure.

So as a concrete example, let's say you want to know how many shots you can get out of a 4500 PSI, 68 cubic inch HPA tank the paintball guys use. Your launcher is fired at 100 PSI and has a 3' chamber of 2" schedule 40 PVC. If you go here you can see what the ID of that happens to be (2.047").

To determine the volume of your chamber, use the area of a cylinder equation: V = pi * radius * radius * height. So:

radius = 2.047 / 2 = 1.0235

V = 3.14 * 1.0235 * 1.0235 * 36 = 118.415 cubic inches (rounded)

This doesn't take into account fittings, elbows, the valve and the like so it's on the low side. Depending on your configuration, you will need to pad this or calculate more.

-- the air tank

pV = 4500 * 68 = 306000

-- the launcher

pV = 100 * 118.415 = 11841.5

Number of shots = 306000 / 11841.5 = 25.84

Not too shabby, though each shot is a bit expensive when you count refilling. Let's try a different example but use the same launcher. Say this 11 gallon, 125 PSI tank from Harbor Freight.

First we need to get all of our units the same. I let Google do the work for me. Just search for "11 gallons in cubic inches" and it will tell you the answer is 2541. You can see this here. So if we run the math again, it looks like this:

-- the air tank

pV = 125 * 2541 = 317625

-- the launcher

pV = 100 * 118.415 = 11841.5

Number of shots = 317625 / 11841.5 = 26.82

A tiny bit better, but significantly cheaper as you could use your own air compressor at home to do the filling. This does NOT take into account the tank cooling as you fill for each shot. In the HPA scenario, this could add up fast. In the larger tank scenario, this is a negligible difference. I hope this helps people out a bit in calculating their shots.

Thanks!

--Mike

P.S. Based on this math, you can see that the 10 gallon, 200 PSI tank here is a MUCH better deal at $34.99 with 39 shots available.

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