I have made an attachment for my airsoft launchers with 10mm OD barrels, it just has a sliding section and a hole to insert the foam ball. It is 3/4" Sch40 PVC, 20mm ID. The balls are Nerf Rival and are 23mm OD, so a nice airtight fit. I fired a couple at 80psi and even with all the dead space it was pretty impressive, the nerf dart not so much, it blew the foam head off as it left the barrel
This is possibly as much fun as you can have indoors with your clothes on
