Yeah, zephyr grooves, but you need a port (i got a few fps extra when I tried this, but not a lot)...

Mote is a 2mm thick 25mm OD washer stuck in the end of the comp tube, with a nicely radiused port, 4mm should be fine. Radius can be quite wide/deep as the port vol is so low.. would only be around 0.5mm of parallel sided port in there (after the entry porting).