There aren't any obvious areas for wheels, from what I can see, and that is a good sign. Hopefully the box will prevent jams and the barrel seems long, which might be a sign that the darts can at least go through that at a decent speed.
If it's an adaptation of another gun, why'd you say it's like no other? Give the guy who made the original some credit. I hope the guy doesn't mind me posting a pic of his gun, so that everyone can see his first version:
At that size couldn't you add more barrels? Im sure you could at least squeeze 2-4 more barrels in there. Great job at making a locking mechanism for the turret. That was always the weakpoint of homemade turrets and lowered the potential ROF, but this solves it.