With some modifications to this gun, you could add a catch to the pin so that instead of going back and then forward all in one motion, it would catch on its way back and stay there, firing when the trigger is pulled, and resetting when the trigger is released.  Of course, it does require you to wind it every couple shots, but it is a resetting pin.  I think I may revisit this concept.

Hard to do that without an additional energy source. "Real" automatic and semiautomatic weapons use part of the energy from the cartridge's grey powder to recock themselves. A knex gun doesn't have that to work with.

Best suggestion I've got is to design something like the rubberband gatling guns, where all the energy is pre-stored and just released in a controlled sequence.