I wouldn't recommend changing anything permanently. The designers did an excellent job balancing every aspect of the game except that damn pistol. If you change anything at all on multi maps, you can't play with others on system link. Even taking the scope off the pistol or changing the zoom on the sniper rifle will make your system link games hang up.
But if you must change it, here's how:
1. The game must be running off the hard drive.
2. Find out what the name of your map is in Halo/maps/. For example, Blood Gulch is bloodgulch.map. This isn't always the case, as there's one called "beavercreek.map" in there.
3. Edit Blood Gulch's cache file (cache003.map in my case) and compress it with Halo Map Editor. You can't change the original file size or it won't work right. Like you can change physics and stuff, but adding weapon spawns makes it more difficult. It's possible to compress it right if you add stuff, but I'm not sure how.
4. Save the compressed file as bloodgulch.map and stick it in F/games/Halo/Maps on your hard drive. Or wherever it is in your case.