Oh, I am sorry. Are you playing on the Master Chief Collection or the original PC version? If you're playing the original PC version, may I interest you in the Custom Edition version?It's free and uses the same CDKey that came with Halo. IT was actually made by Bungie for player that wanted to do customizing. In addition to the original single player missions, there are a few custom campaigns that are a TON of fun, not to mention there are a few REALLY fun cheats, like bottomless clip(infinite ammo once you have a weapon) and and one that enables an infinite cloak, which is greatly fun for sneaking around!
If you're interested, PM me and I'll guide you through finding and installing what you need. This goes for everyone as well.