that looks cool as hell, I will put that on my "to do" list...lol!
My uncle built me one similar to this for me when I wasr around 12 they are tonz of fun.
You can use a NON- deodorant aresol anti-prespirant (right guard) as propellant also.
The deodorant ones will gum up the firing mechanism.
We used a flint striker from a coleman lantern.
We used aerosol hairspray and the colman flint striker and our combustion chamber was considerable smaller. After watching this a little closer I noticed he didn't use primer before the cement, I dunno how important it is for a spud gun but I was taught to always use primer first when cementing pvc. If you build one do be careful who you shoe it to, turns out they are illegal to even own in the city I live in.