FreeGameDev

Yes, you can do all of the above.

RE allows you to host your own server with the dedicated server files, which you can use to modify the weapon vars and change them dramatically in almost every facet.

You can then edit the maps and add more weapon spawns, or make your own maps with your friends online.
You can then package these and upload them to these forums for others to download and play on your server, I do all of this for Rou's Madhouse.

The nice thing about the weapon vars is people don't have to download anything, they can just join your server and play them without having to download and install your mod.

You can rather edit the properties of any existing weapon than adding new ones to the game. A majority of the weapon related variables can be edited via the games main menu, just hit variables, then select the [weap] tab. To get an idea of what you are doing there, check out the wiki:
http://sourceforge.net/apps/mediawiki/r ... eapon_Vars
It won't be hard for you to find the fullauto variable there.
There are several topics that explain how you share such simple modifications. I don't need to point at mine, do I?