You've never used these guns and or played these levels in any Halo game, and odds are, you never will.
The latest Did You Know Gaming video delves into darkest, strangest portions of Halo's history, the most interesting of which -- in my opinion -- center around cut content from the master and chief of alien-blasting's earliest iterations. The original Halo was once set to be a very different game. Looooooong ago it was meant to be an RTS, and later a third-person shooter -- not an FPS.
Even after that it was going to feature weapons like a machete, a spear gun for hunting underwater enemies, and something called a "gravity wrench." Oh to play a Halo game where Master Chief is a grizzled space captain endlessly pursuing a legendary space whale so he can beat it over the head with a gravity wrench until it dies. Maybe someday. I can hope.
Early envisionings of Halo also included a whopping 25 levels, many of which would have had fully functional weather systems and indigenous lifeforms prancing (or whatever verb you use to describe rhinoceros velociraptors) about. It was, in other words, set to be even more of a pseudo-sandbox than the original Halo ended up. A tantalising, albeit probably chaos-prone idea.
Other noteworthy tidbits in the video include countless connective fibres between Halo and Bungie's previous sci-fi shooter opus, Marathon, and -- my personal favourite -- the true origin of all the weird noises Elites made in Halo 1. Turns out, they're slowed down and reversed snippets of Sergeant Johnson saying things like "Go, go, go!" One of my roommates in college was a Halo nut who would have eaten other Halo nuts for breakfast if he were a cannibal, but I don't think he even knew that last thing.
So there's your Halo history lesson. Remember: much like guns, knowledge is a weapon. Use it wisely, or to harm people you really don't like if you think you can get away with it.