NO. The fit is perfectly. Think about it. The seal between the suction and pipe is perfect... shove the whole dart, not just the foam into the barrel. Yes, even the Suction cup.
Ah, I gotchya. So if you want to use stefan darts you don't want to use just a 12" PVC barrel because that would only work with stock darts. Is that right? And if you want to use ONLY stephan darts, the best thing would be to use a shorter piece of 17/32" brass inside a longer piece of 9/16" brass (assuming you're using 1/2" FBR)? Is that right?
But what if you want to be able to use BOTH stefan darts AND stock darts in your gun?