[falconmaster]

I think I can answer everyones questions. We just made a ABS , 4 inch, tank with two endcaps. In one side we put in a threaded coupler connected to a ball valve, which had another threaded coupler that mated to the 1 1/2 inch grey pipe. We only used 60-70 psi. The projectile was a smaller pvc tube , 8 inches long, with two end caps that fit pretty close to the inside diameter of the grey pipe. Inside the projectile we weighted the front end with some bolts and stuffed the back end with foam. It was front heavy. This kept it going straight through the air.
I guess we did shoot 80 yards, forgot the endzone! We could have used more psi and shot farther, but alas no time to practice before the one shot we were allowed.
The stands are full! At least for Football at our school.

The whole trick with the air gun is to get the pressurized air out past the valve as fast as possible, only then does the gun work well. A quick wrist is essential!