Part of building a connon has to be the thrill of how damn funny it is to shoot the thing. The thrill of the X factor. The X factor is the idea that you have really no clue as to what is going to happen the first time you pull the trigger! It could misfire it could need igniter adjustments it may, ( very very very very have i said very? Very RAIRLY come apart upon combustion. So Ok you have the parts to build the thing... BUILD IT AND FIND OUT!!! THATS PART OF THE FUN! In my opinion, you'll be just fine sched 40 is what I use every time unless its a metal cannon. You should be AOK! You'd be surprised how much pressure a cylinder of plastic can take! WORST CASE scenario, ot blows the barrel off or apart. Big dean its not like your using gun powder...
BUILD IT FIRE IT AND LAUGH YOUR ARSE OFF!