Pretty Awesome
It's good in so many ways. I love how you actually used the right term and used magazines and not clips; that made me really happy as a lot of people interchange the two when they are completely different things. I also love how you made it so you lose all remaining bullets in a magazine when you reload; that makes those of us who are reload freaks to hold back on our 'R' keys.
One thing I don't like--and I understand that it's part of the style and it's easy to make the sprites--is that the enemies are stick figures, thus, making head-shots easy and body-shots difficult when the reverse is how it should be.
Also, having a more realistic combat system would be nice. People usually avoid being shot and killed, so they won't stand strait up in the open (or hug walls in a corridor); they will either make themselves a smaller target (usually by crouching), or find some cover. And if they're being shot at, they will stay in cover until they feel that it's safe to come out. The parts where S was able to choose when to hide and when to come out and shoot were great.
I know that these things are difficult to implement and this is a free, online, flash game.
I'm not saying this game is bad, in fact, as it is, it's really good.