I repeatedly fought Dracula in the Boss Rush, shooting him slowly so I could count the shots. Here are the results:
Trial# | bullets shot at a time | hitpoints result (stage 1-2-3)
----------------------------------------------------------
1 | 2-3 | 13-19-1
2 | 1-2 | 13-16-1
3 | 1 | 13-15-1
4 | 1 | 13-16-1
5 | 1 | 13-18-1
So you guys who keep saying it takes 50-60 shots to kill Dracula are very wrong.