Jason jx - get HMT - Halo Map Tools 3.5 - you can edit pretty much everything and its very easy.
All you have to do is load the level you want, lets say assault on the control room. Once the level loads, reboot the xbox, and leave it at your evox menu. Now you need to ftp to your computer. Once you have ftpd to your computer, search for your cache file, I believe mine is in the Y: folder, but it may vary on different xboxs. Anyway you will see a bunch of .map files such as cache000.map cache001.map cache002.map cache0003.map and so on. Now FTP the cache000.map to your computer. Once it is transferred over, open it with HMT - halo map tools 3.5, and start editing all the vehicles weapons you want. When you are finished, make sure you save it. Then FTP the file back onto your xbox making sure you replace the existing cache000.map file. Then boot the game back up for the same exact level - assault on the control room and it should boot all the edits that you did. Make sure not to go crazy with the editing though - some people tend to get errors or the level wont load due to too many changes.
Download HMT 3.5!