Here's my latest project, a pretty simple pneumatic mortar with semi-automatic capabilities if I ever get around to making a loading mechanism. It was built for lobbing large projectiles fairly long distances or as a mortar to rain down airsoft BBs or paintballs.
Specifications
CHAMBER: about 2' of 1-1/4" PVC pipe
BARREL: about 2.5' of 2" PVC pipe
FIRING VALVE: 3/4" QEV
PILOT VALVE: 3/8" slide valve
NORMAL FIRING PRESSURE: 60psi-160psi (I might go higher with a metal chamber if I get a fridge compressor)
AMMO: Clementines, NERF pocket vortex footballs, BBs, PBs, Water
FEATURES: Backpack CO2 system, fully adjustable/folding bipod, fishing reel so I don't lose too many NERFs
Side View:

Front View:

Back View (sorry it was setup wrong and I didn't realize ATM):

Fully Folded for Carrying:

Many upgrades to come, but thats all for now