SpudFiles

I am currently working on building an air gun, and I just wanted to get some advice, and a few more pair of eyes on this project before I take it all the way. The majority of this gun will be made of copper piping, except for the barrel which is a half inch galvanized pipe with an inner diameter of .50
I am currently deciding on whether to make the body of this air gun out of 3/4" or 1/2" copper pipe. The half inch is rated to 722, and the 3/4 is rated to 580. Would a 1/2 inch air receiver be a large enough diameter, and how long would be the recommended length for the air chamber?
Everything is going to be soldered together with 95-5 tin antimony. My ultimate goal is to charge this thing to 500 psi with a HPA air gun pump, however I am going to work up to this pressure and will use a regular air compressor in the mean time.

I drew out my build plans as best I could, please take a look, any input would be greatly appreciated.

Picture isn't loading for me, but I have a reasonable idea of what you're trying to achieve.
As a general rule I follow, air chamber as big as you can get it, I know a lot of people will talk about diminishing returns and increased fill time, but that aside, it's not going to be detrimental to performance if you make it bigger.

The bonus here is if you are working with copper you can always cut it down. heat apart then reattach any components on the end, always easier to cut down rather than make it bigger.

Sorry, I have never uploaded a picture before so I might have it in the wrong format. It is the go to design that I have seen a lot of people using, with the air chamber tube running under the barrel, sort of a U shape, with the valve going out the opposite direction of the barrel and air chamber.

Thanks, I hadn't even thought of that, I guess I will make it as long as I can, and cut it down as I see fit. I am excited to see how well piston valves work, thanks for the input. One last question, how much air needs to released for the piston valve to function? Can it be a small amount or does it need to be sizable?

The smaller the better with regard to that! That volume of gas in spudding terminology is referred to as the 'pilot volume' the smaller it is the faster your valve will open, but don't go overboard.
My advice would be to copy the size of some existing designs, then it makes it a lot easier to troubleshoot