1. There could be flywheels added to the gun so that the darts are chambered by the air and then they hit the flywheels to go much faster. I used a similar concept from an old Hot Wheels-Cyborg Assault playset when I was still young and innocent. That thing shot full-metal Hot Wheels cars about thirty-five feet at a speed enough to knock out a good hole in a cardboard box. Mind you, we're talking some high-speed shit and this thing took six D-cells (as everyone knows, the same as the Stampede and Vulcan) and it had motors about two inches wide and flywheels three inches across. They wrre also rubber-coated and fit the cars really tight. But I don't have it anymore (Bye-bye, Hot Wheels cannon, I'll always miss you). But the flywheels could add a lot of range to your design.
Also, a different magazine would be a nice idea. A custom detachable box magazine could allow you to not only switch ammo very quickly, but if built like Boltnsniper' FAR magazine, doesn't require gravity. It would also free up space on the top to attach iron sights so that accurate bursts can be achieved.
But that's just my FNG opinion.
USER WAS WARNED FOR THIS POST
As per the Code of Conduct, it's best to not post in threads that haven't been touched in two (or five) years. Suggestions don't fall under something "significant" to add.