When I was 10 saw a spud gun then 2 years ago googled for plans(I was 13),used crappy plans fixed them and now I want more power

So I'm 15 and am in the middle of nowhere.. I've done spud guns before out of sch40 abs but am wanting to go bigger better and think it would be cool to have a somwhat economical hobby (compared to reef keeping) on the side. So I'm wanting to do a 6 foot combustion thats ABS, what are the optimal di...