Here is a 300 psi rated air chamber that is very light and affordable . Made from a fire extinguisher body. Run them in all my cannons. The RPG runs 150 psi even though I did a trial at 200 psi that did not significantly improve performance. I run a very short 20 inch barrel and the nerf is out of the barrel before the valve gets completely open. So no improvement. Like 10 fps for a 25% increase in psi.
I use the same in my tank but it is a walking tank where the cannon configuration is essentially the same. Small, compact, light.
It is 100 cubic inches in volume. A good rule of thumb to use if you want BOTH type of velocity cannons.
I hope your not using PVC. Many fields do not allow it. An increasing number of field allow only commercially manufactured RPGs. They tend to be more flexible in talking about tank cannons.
Here is a handy link. A air chamber volume calculator.
For example the 2" x 17" calculates to be 53.41 cubic inches. About HALF the volume I use.
Be SURE to use the RADIUS and not put in the diameter of the tube.
So far, you have not gotten enough information to make any decisions. You need answers to several questions.
Some games limit RPGs to 150 fps while others allow 230 fps or more. Saw one field with a 250 fps limit. So which one are you building for or do you want to be able to do BOTH.
You haven't said this is a RPG or tank cannon. That makes a difference because players want smaller, lighter, more compact handhelds compared to one mounted in a motorized tank. Completely different set of parameters and design.
Four major interacting factors in cannon design.
1. Air chamber volume. Lower psi cannons need larger volume. Higher psi cannon can us smaller volume. Same air charge expanding into the barrel.
2. Valve. Sprinkler and Quick Exhaust seem to be most popular. Perform about the same. Plastic vs All Metal valve. In all cases, the valve has to have TIME to open completely. Time for the Nerf to be accelerating in the barrel. Longer Barrels give you MORE TIME for the valve to open and give you the flow rate you need to generate the velocity. Barrel length greatly affects velocity of the Nerf.
3. Barrel Length. One way to have BOTH velocities. Just have interchangeable barrels. The longer one gives you the higher velocity. Start long and cut off 1" sections until you reach the right velocity. That way you do not have to use the more inconvenient way which is to adjust the operating pressure.
4. Operating Pressure. Higher pressure = more velocity but with limits determined by the valve flow and opening speed. Higher pressure = more risk for something to go terribly wrong.
RPGs are particularly vunerable to being damaged during play. I have fallen down on top of mine, banged it on bunkers, rocks, trees. A cannon in a tank in is a much more protected environment and if something should go wrong the most likely victims are the occupants inside the tank and not players on the field. That is WHY I went with the all metal valve in my RPG.
Link for the valve at: http://www.pbnation.com/showthread.php?t=3503911