This is a simple remix of Kprime's Nerf N-Strike Elite Firestrike restrictor replacement ( https://www.thingiverse.com/thing:2271647 )
The only thing I have changed is to add 4 tiny dart stops, in order to prevent vacuum loading, as the hole is big enough for a dart to fit down, and with only a brass barrel this can be a problem.
The stops are small enough to not interfere with airflow but just big enough to keep a dart from entering the plunger tube.
I printed with no supports and didn't have a problem