OOC: Ah, doing some research.
The conventional mortar shells have a safety pin which locks any kind of reaction from happening. To arm the mortar you use a kinetic force (usually generated by actually firing the shell from the mortar gun, smacking together would work as well as long as you smacked them where there pressure plates were, which is usually the base of the mortar shell). After this event it arms the mortar shell and of course any kinetic force after that will detonate it.
However, that's only for WWII mortars, modern day mortar shells use a much more advanced system, some even have laser guided sensors and timers to detonate the mortar shell just before it hits the ground, showering the target in red hot metal fragments. Either way, you were right, I was wrong.
(Back to character:)*Smacks shells together then puts them in the catapult then throws them*