Me and my cousin have just made smallish sized spud guns. Just recently the cheapo ex- lighter ignitors just stoped working (Not designed for many pushes. We tried a lot of non grill ignitor or flint methods. As in NZ I can't find either of those. I found an ignition coil in the garage and though 'Why not?' But the sparks sucked. I hooked a camera flash to pump the coil and CRACK! massive thick spark. If you look through the pictures you will see how easy this actually was.
EDIT:- Added new photo detailing the circuit
Also if you are going to use this please wrap your gun in something (Duct Tape, Fibre Re-inforced Tape, Denim...) This spark could ignite a thicker mix than a grill ignitor and we dont want any explosions. Unless you have shedule 80 - 120 please dont get excited with the propellent.