The thing is Zero might shoot fast but sonic isnt only fast he is MUCH faster ! Also the speed at what Zero shoot would actualy affect his aim. Sonic is agile and fast he could easily just run away and gather speed, then he would just need to run toward Zero dodging attack from his gun. In no time sonic would get behind Zero, then Spin Ball and Bang ! Zero wouldnt even have the chance to react sonic speed would just be to much and if that wasnt enought he could always repeat since the begening (gather speed run to Zero ... behind or side then BANG).