that is very cool. a great job, especially for your first! i love the idea of pneumatically actuating the 2" ball valve. the articulating arm on the piston is a particularly nice feature. is it a pvc valve that you are using, or brass? the only problem i can see is that the thing really isn't too scalable. this would be incredible for a platform gun, but i don't think it is too practical for the average backyard spudder.
just out of curiousity, how long is your barrel? also, why is there a tee on the end of it? (at least, that's what it looks like in the third picture.) is the bottom of the tee cut out, to give the barrel a tank cannon look, or are you planning on hooking it into multiple sub-barrels?
the only aesthetic problem i see is the piece of lumber holding the barrel up. you could replace that with a length of PVC and grind out an arc into the end of it so that it matches the rest of the gun. or maybe use two metal rods that lead down toward the ends of the chambers. it isn't a real problem, just a small incongruous part of a very nice looking gun.