
Board: Code Library
The trigger you posted won't work reliable because people us helpers for attacks on other things besides projectiles and projectile sctrl is used for thing outside of projectiles so that could trigger at times that you wouldn't want it to. Also projectiles sctrl is always owned by the root, even if its called by a helper so the playerid redirect won't work since it points at a helper.
The best case for a projectile in my opinion, is to have the helper use its own hitdef but also call a projectile sctrl with a blank animation, using the helpers x and y and removing it every tick, no vel needed. This way AI without these systems can still check for a projectile and AI using this system can also check for a projectile easily.