i know its a crappy picture, all i have is paint...
The way it works is you have a standard sch80 barrel that has some sdr 21 sleeved in it with about the last 4'' cut off. So if you have 30 inches of sch80, then you only have 26 inches of sdr 21 sleeved in it. the last bit of sch80 is all by its self with out sdr 21 in it.
you load the ammo into a 4'' section of sdr 21 and place it in the last bit of sch80.
along the last 4 inches of the sch80 pipe, there is about 1/4-1/2 '' cut out of it as a groove a that is about 2'' long. this is because there is a rod or lever that goes in the shell to help pull it out.
does this make scence? do you think it would work?
click image to make "big"

ps- what is a good program for drawing 3D images?