.qc files are control files from milkshape's md3 exporter. They're used to store the texture path within the .md3 at least from what i recall. If you are not using milkshape then no need to worry about that. With max, your exporter will usually store the texture path in the file. If not, you can write the path with the md3 tool provided with the .sdk.
Textures for the guns should be easy to find (just extract the weapons dir and you'll see) Even if they are named as .tga in the .skn file, most of them will use .jpg, it's a programming thing.
Tags can be used to tell the engine how to bolt your object in the player's hands or simply tell where the "firing" emits from... also called flash_tag or something similar.