Alright, I'm kind of new to this board, so I'll try to be as professional about this as I can. I only know how to make such devices, the science behind it is still a little hazy. So if I make any mistakes, feel free to criticize.

I've recently made my first pneumatic spud gun and it worked nicely. That was when I realized that air rifles and pneumatic spud guns work on the same principle and have the same five main parts (pump, fill valve, chamber, release valve and barrel). Now I thought to myself "is it possible to downsize a pneumatic spud gun that it may operate like an air rifle?"

At first glance, it would seem that it is very possible. Using a hand pump (which displaces a very small amount of air) to pressurize a small chamber that is still 4x larger in volume than the volume of a barrel. Because the chamber is so small, even a hand pump can get the chamber to high pressures quickly (similar to a multi stroke air rifle).

Lets say that I am going to make my barrel diameter 6mm (the same as an airsoft round), whose length will be, say 600mm. My bicycle pump displaces 70000 cubic millimeters of air in one pump.

So the volume of the chamber would be:
radius^2 x pi x length
=3^2 x 3.14 x 600
=16956 cubic millimeters

If I want the chamber to be of a good 4:1 ratio with the barrel, then the volume of the chamber should be about 16956 x 4 = 67824 cubic millimeters. For ease of calculation lets give my gun an overkill in pressure and say the hand pump displaces the 67824 cubic millimeters (reduced from 70000) which is the same as the chamber.

If the average air pressure is 15psi, if I were to pump once into the chamber, it would be 30 psi inside right? And for every time I add the same amount of air as the chamber to it, it increases my multiples of 15 right? So in 9 pumps, I should be able to get 150 psi of pressure in the chamber. This is still FAR less than pressures in an air rifle, but for PVC, that seems like a safe amount ( I could probably safely do a few more pumps, but because I overkilled the pressure before, I want to be safe).

So heres some questions about this:

1) Is that pressure adding method I used above correct?
2) Is there a formula to calculate acceleration, velocity and ultimately, force, if I know the pressure and other things I can measure with good old measuring tape?
3) I do have a spring piston air rifle, but not a multi stroke one. For people who do have a multi stroke, how hard is it pump 10 times in succession?
4) Would you predict that a bicycle pump rigged like this would have the same difficulty in pumping?
5) If an air gun was a smoothbore, and fired airsoft pellets, should it have similar performance to this minature spud gun if the chamber pressures were the same?
6) Other than higher pressures, stronger materials and a rifled barrel, are there any other differences between a spudgun like this an a real, multi stroke air rifle?
7) Are there any different valves that I should use that would suit a smaller barrel?
8) Just an unrelated question about design, are there any "pistol grip" type release triggers? It may be a fun thing against rats etc.
9) Another design question, I know there aren't any good breech loading methods for such a narrow barrel, so would a pipe whose bore diameter was the same as the barrels outer diameter, lined with a melted rubber for a better seal that slides over the breech be good enough for such high pressures? Remember that the breech is a part of the barrel and is after the chamber.

Sorry if theres too many questions. I'm kind of a beginner at this, so feel free to leave constructive comments, criticism, or flames.

There are formulae to describe and calculate anything. The ones you'll want to use here are PV=nRT and some geometry formulae and some simple physics like F=ma, and perhaps a few gas laws. You'll also want P=F/A.

However, these things are for ideal environments, so your calculations won't mean much unless you REALLY want to get to the nitty gritty. You'll have to take things like incomplete reactions, leaks, friction, and other stuff into account.

Basically, pneumatic spud guns are fun, but what is a ton more fun and easier is a combustion one.

Well, I already have a few combustion type spud guns, I'm just looking into the challenge of replicating an air gun or at the very least an airsoft gun.

What you want is the gas gun design tool. Nice little application. Can Be found Here: http://thehalls-in-bfe.com/GGDT/