JSOC has tried tested and proven mortars, fires super simple, easily built, VERY low cost disposable rounds. Has a range of 600+ feet, light enough to throw over your shoulder and bugger off, and it runs off either 2 bike pumps (two for speed) or an air compressor. Even with one bike pump it doesnt take long to charge. They're extremely durable and waterproof, and pretty accurate for what it does. Not to mention sounds awesome from the recieving end.
We've also make short rocket launchers based on the same design, fires either 2" nerf rockets or the same rounds our mortar uses.