It's because mta is client-side,so u shoot first and your game assumes to be hiting,but the person's game(The 1 ur shooting)doesn't assume,therefore he shoots u on his game and hits u,and the same happens to on ur game,therefore both get stuck in firing.Btw try to shoot first then other people  if you follow death's advice,if theyre shooting to ur right and u shoot to their right,they hit u(They keep aiming to the same side,but....  )