all you have to do is put a big spring in a pvc pipe and have a catch lever cut a 2 liter bottle in half fill it full of bbs and throw the lever and bam you have a airsoft mortar
actually, springs big enough to power a mortar are very hard to find. i made one not too long ago that was air powered, and it worked great, there is a thread on it and everything
trowing bbs into the air does not count as a mortar a mortar is something that shoots a mini or full size vortex football etc. i have plans to make one that will be able to shoot on about 400 meters(very very far-WOOT!!!) and also closer if need be. i want to nail them across the field