No batteries required at all! Can be full-auto, semi-auto, or burst.
O-rings can snap very easily when installing.
It's an amazing trigger system, no use of batteries at all, I love it. The best feature about it is that depending on how much pressure you put on the trigger and for how long you hold the trigger back, you get different firing modes. You get semi-auto, 3-shot-burst, and full-auto with different pressures! Even the price on it is amazing (depending on which retailer you go to).
I definitely recommend this to anybody that has a Tippmann marker and doesn't want to use batteries. It sure as hell beats having a stock semi-auto trigger system too.