Generally speaking, there isn't much difference between ABS and PVC pipe. ABS pipe is used for non-pressurized drains and I have read articles in trade publications regarding a tendency for ABS fittings to crack. However, these were mostly anecdotal and not scientific studies. PVC pipe and fittings are pressure rated for fluids and must conform to specific ANSI/NSF standards for pressurized systems. For this reason, I would choose PVC for a potato gun, using PVC with a high pressure rating, say Schedule 80 or 120.