Automatic bb guns are not designed for maximum power, but for maximum fun. You will find that the majority of the available airguns offer between 250-500 fps (feet-per-second) which only translates to a maximum of 2.83 fpe (foot-pounds-of-energy). One option on the market is the Hatsan Blitz full auto pellet gun which can get up to 53 fpe in the .30 caliber configuration. But with any option, whether you use a full or semi automatic bb gun you sacrifice accuracy.