I know there is no switch, but (sigh) if you set the TPUL and the TREL to 001, gas it up, and find the right level of tension, it will seem to autofire. I believe it is just a matter of the marker moving around enough that the trigger (standing still on your finger) trips the beam repeatedly without effort or conscious movement on the player's part.
It was easy enough to do that I set my own filters a bit higher in case I ended up in violation of a rule, in the chrono ref's opinion. Not something I'd want to mess with before a game, though it is easy enough to adjust.