Just in case a picture helps...
The selection of bullets will be a 'snake' like this one, and the question is how many possible snakes are there to choose from.
For each of the 10 possible choices of firearm for bullet A, there are 10 possible choices for bullet B - so far that's 100 choices. And so on.
Does that help?
Don't integrate - balloontegrate!
Balloon Calculus; standard integrals, derivatives and methods
Balloon Calculus Drawing with LaTeX and Asymptote!