You start with a 4" ABS fitting, its the y fitting. this is key- the 4" has a 2" y coming off it. the sole purpose of this is for a handle. buy the necc. fittings to plug the 2" handle, and the fittings to plug the back of the 4". but you will need the screw in piece on the back. this is where you unscrew the back to load the fuel/ air mixture. In the middle of the screw in fitting, drill it out to put in one of the barbque push button ignitors. You now have a 4" abs chamber, plugged on the back with a screw out fitting to load, and with a 2" Y that is plugged as the handle. now for the barrel. reduce it down to a 2" female on the front side of the 4" ABS Fitting. now you can build more that one barrell, and just screw it in.
Ive launched massive amounts of spuds and have it down.
1 1/2" barrels that are short will launch smaller spuds quickly, and are very loud, like a gun.
a 2" barrel- short is going to give a loud quick shot to a larger spud
BUT- a 5-6' barrell, 2", is perfect! the shot is full force and nearly silent. You get this huge WHISHSHSHshshshs. out the end of the barrel. want to play with fire? then add a little oxygen shot- A VERY LITTLE- to each fresh load. this is available if you have oxygen accetylene welders. Maybe nitrous would work.
also- blow out the barrel each time with fresh air. best accellerant is aqua net hair spray. WD40 is good but tempermental. I heard MAP gas is good too.
Caution- these are very dangerous and can do massive damage to a body! We shot one from the inside of a room, through the sheetrock and the waferboard and the lap siding of a new house.

massive power.but i look at it this way, there are a lot of things that can be dangerous if not used responsibly. like a gun.....