For about the first 5 or so darts everything goes fine but eventually one dart will get stuck halfway between the barrel and the belt and it'll jam the whole thing up. What I have also noticed is that when it does actually fire the dart, about 1 in 2 darts exit the barrel significantly slower than normal.
Based on these two things, what I think is happening is that the plunger tube isn't making a good seal with the belt or the tube is retracting too early.
Another possibility is that the clutch in the belt gear is slipping, causing the belt to lag behind. Currently it's running on 12V from a power supply that can provide up to about 3-4 amps. I had run the Vulcan stock at 12V with the original spring and it worked fine so I doubt it's the slightly increased feed speed might be causing slippage.
So I was just wondering if anybody that's had more experience diagnosing Vulcan issues could provide some insight into what's causing this.