Nice!
This is a cool game with some neat ideas going for it. First and foremost, I really like the way you have the weapons set up - the advantages and disadvantages of each one make for some real thinking (believe me, I was alternating levels with my girlfriend, and we disagreed on what path of action to take on more than one occasion.) The turn-based system is pretty good as well, it makes you consider whether or not you really want to get the character close to a bunch of enemies to get the mass kill, or risk exposing yourself to easy headshots and grenades. The challenges are pretty fun too, although they get hard way too quickly for me to try anything beyond the first couple.
However, there were some drawbacks that somewhat pulled the game down. Obviously, the AI was the biggest problem - I know this is your first time writing it, so I can understand this. The enemies have way too much of a tendency to try and shoot you through walls/floors/etc. It seems that as soon as you're within a certain proximity of them (but not standing directly on their toes), they go for the gun regardless of whether or not there's a floor, or wall, or even another teammate in their way. Also, the setups were a little uninteresting - except for the King of the Hill one, it was all "all my guys on the left, all the baddies on the right...CHARGE!' Maybe consider more scenarios - you know, being pincered (placed in the middle of two groups of enemies), capture the flag, having to seek down a particular enemy target (you know, say you have to shoot "Bob" on the other team, and Bob's buddies are setup to easily protect him)......etc. There's lots of possibilities, so you could expand on the scenarios a bit.
I still enjoyed this game quite a bit. Hope you plan on making another one or few more.