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Safe, efficient, breech-loading pneumatic tennis ball launcher?

I was interested in building a tennis ball cannon, and have seen breech-loading ones in videos.  Does anyone know of free plans for one?  I was also wondering if there is a disadvantage to ABS other than the slight cost difference.  I'm looking for safety and an efficent barrel-to-tank ratio.  How would one make a resettable pressure release in the case of some sort of accidental over pressure?