It's pretty simple.

There are two sprites. The bullet sprite, in this case just a white circle (it's coloured later) and a black outline. The black outline sprite is drawn a layer underneath the bullet sprite itself, so the bullets will overlap each others' outlines.

:D

Two fives?
:DDD

I'll probably add online highscores and such as soon as the GameJolt achievements and high scores system is publicised ;)