Stan, I haven't seen a potato gun in use for probably 20 years or so, but a friend made one with a barrel of 2" PVC pipe around 42" to 48" inches long, The combustion chamber was made from a 4" tee with a reducer, and it used an igniter from an old gas grill. I never measured the range, but I'd guess it was launching a good sized potato at least 250 yards or better. And when he managed to hit the side of a barn about 200 yards away, it splattered the spuds and made a very loud thump. I'm surprised they didn't break the old chestnut siding boards. It was impressive enough that I was a bit concerned about the chamber pressure that was involved. A potato weighs a lot more than a tennis ball, and I wondered what the burst pressure of those schedule 40 fittings would be.
We also learned that you could dump a little calcium carbide and water into an empty metal 5 gallon tar or paint bucket, and crimp the lid closed. The fuse was a paper soda straw crimped closed. When the acetylene that formed detonated, it would open the bucket at the seam and launch the lid almost out of sight, and the sound was deafening.
The beer can guns were simply noise-makers, and with the cans opened at both ends and taped together to form the barrel, they came apart at the seams pretty quickly if you tried firing even lighter projectiles. The pumpkin chuckers SKB mentions above are very impressive to see too.
To answer RWTF's earlier question about sizing a potato or apple to the bore... The muzzle of the PVC barrel is sharply tapered so that it cuts the projectile of choice down to a perfect bore size fit as it is rammed down the barrel. No wadding is necessary. Sometimes I really wonder how we lived past adolescence.
