SpudFiles

I came up with a very simple way to get real mortar like action that should be legal.

Basically you have your mortar tube with the cap on the end and a glow plug in the center. Then you take your rounds (end cap + pipe section+ coupling) fill with fuel air mixture and cover with saran wrap.
Now drop them down the tube. The shell hits the glow plug and boom you got a mortar! It's the way some reenactors do it. Except they have more potent propellants.

I thought of an idea like that myself a while back, although I never had the funds to make it. I can't see anything wrong with it, as long as you get the right mix then it should work, it's definitely an idea worth trying that's for sure.