Well, I know this is pretty easy to make (well, I made it, so it must be pretty easy) but it does change the gameplay substantially(if you can even call it that after this... I prefer mayhem).
I will post a video preview as soon as I get it all finished up. Some of the features include: The infamous rocket-AK47, minigun-esque M4 and MP5, and an Annihilator with chainguns that shoot a bit faster (and harder).
Installation is simple: Backup WeaponInfo.xml from Rockstar Games\Grand Theft Auto IV\common\data, then place my WeaponInfo.xml in its place.
Currently working on another weapons mod which I will post in a few days time.