This setup seems to be feeding silicone dome darts fairly well so long as they don't exceed 1-1/2" in total length. But I'll have to war-test this to be sure. I've made three of these and one of them got a little mangled in the process of cutting the corner so I cut it in half to make a cross-sectional model.
Red: I used a Burr Bit to cut the inside of the corner inside the Wye. Alternatively a small round rasp, pocket knife (curved blade), or a dremel sanding drum could all be used. Remove as much material here as you can.
Blue: This is a "knife" made out of 1/16" thick polcarbonate that acts as a ramp. Other plastics could be used and the shape really doesn't need to be very thick to work. It also prevents suction-loading and makes backing a dart out through the supply tube much easier. Template The SCH80 stub behind it has a slot cut into it to accept the tabs so that the "knife" can't rotate out of alignment after the stub is glued or screwed in place.
Flute: I drilled out SCH80 CPVC to 5/8", then drilled half inch deep with an 11/16" bit (to match the ID of the Wye). I then used the Burr Bit to smooth the transition between the sizes. Then used a knife to clean up the rest. And lastly I cut the top side at an angle so that it doesn't impede the inside of the corner.