yeah it's not a nerf forum...
by allowing only PVC cannons you make things stay as they were in 1995... I don't know if that was the idea really
no restrictions would result in
better designs... those who want to use PVC would have to be more creative in order to build something that can compete with metal guns while those who use metal would have to find some way to deal with increased weight and price...
if you really need to balance things out (but IMO that makes more sense in an online game or something.. not in real life
) you can add weight restrictions - for example max weight of the gun has to be lower than 15 or 20 kg