This is a relatively simple calculation to figure out how on earth your going to make your chamber 1 1/2 x's the volume of the barrel.  Don't ask me why this is the case.  All I know is that is the best barrel chamber ratio for a combustion gun out there.  The text in blue is an example.

(1) Determine which Pipe sizes your going to use for your chamber and barrel. (e.g. 1 1/2 in. barrel : 3" chamber.)

(2) Determine the volume of your barrel (e.g. lets say that you want your barrel to be 36" long. So that's P×r² × 36 (3.14 x 0.75² x 36 = 63.585 in.³  )

(3) Now multiply that by 1.5 (and you get 95.3775 in.³)

(4) Find the area of the base of your chamber - P×r²  (3.14 x 1.5 x 1.5 and you get 7.0875in.²)

(5) Now you set the equation ( 63.3775in.³=7.0875in.² X )

Then you simply solve for X by dividing both sides of the equation by the # next to X and if you did it right you end up with the length of your chamber (13.5 in.) 

So that's a 36in barrel of 1 1/2 in. PVC and 13.5 in. chamber of 3" PVC.  This is the optimum barrel-chamber ratio for your combustion gun.