You could always just build (tinker) with a stiffer spring in the cutoff lever assembly.

Pull the trigger, fully to the rear, as fast as you can.
Does it fire single?

I use a 4000mah 11.1 lipo (The brick) in my guns, in some it will fire 3 rounds if I pull the trigger slowly all the way back. It is that fast and fires that quickly before the cutoff lever engages.

If I squeeze it fast, it does not do it. I can 3 round burst as needed, but not recommended :)