Halo 2 has specific button combos that, if executed right, allow the Battle Rifle to glitch and shoot super fast. At the level you’re playing at I wouldn’t worry about them, even at high levels these are situational.
Halo 2 and Halo 3 let you dual wield specific weapons like SMG’s and Handguns.
In Halo 2 the firepower remains the same, so you essentially get twice the power.
In Halo 3 dual wielding is significantly nerfed, you suffer damage/firerate penalties when dual wielding (I forget the specifics). In some cases its only slightly better than using one gun, and in others it’s actually worse (Magnum).
You can also no longer dual wield Needlers in Halo 3.
The Needler is really only an effective weapon in Halo 4, Halo Reach, and Halo 2 (when dual wielded). Otherwise you would be best off forgetting it exists.
The Assault Rifle is also another generally niche and useless weapon. In Halo 1, it’s only good for when you’re in CQC (or if you’re the one guy on the team with camo, as camo resets after firing easier when using the AR). In Halo 3, it’s bested by the BR at medium range and the SMG at close range.
Utilities are the most versatile weapons, and what you should be using the most often.
In Halo Ce it’s the Magnum.
In Halo 2 and Halo 3, it’s the Battle Rifle.
The Plasma Pistol locks on very little in Halo 1, a lot in Halo 2, and less in Halo 3. In Halo 1 its best used by spamming, as it can slow down targets in this game and the charge shot is unreliable.
In Halo 2 the Rocket Launcher can lock on and home in to vehicles by aiming down the scope and holding the scope on the vehicle for long enough. Halo 1 and Halo 3 do not have this. Halo 3 has the Missile Pod instead of this feature.
Vehicles can’t be destroyed in Halo 1. You either have to kill the occupant, or flip the vehicle over to force the occupant out of it.
In Halo 1 (the console version) your bullets fire out of your gun, and your gun trails your crosshair. Meaning if you are aiming really fast, the gun can lag behind the crosshair and not shoot where you are aiming. It is unclear if this will be fixed in the MCC or not.
In Halo 1 and Halo 3, bullets have travel time. Every other game, they instantly hit what you’re aiming at. This means you have to lead targets to compensate for range (and in Halo 1’s case, range AND other factors).
In Halo 1 frag grenades don’t bounce very much and take a significantly longer time to explode. Though this also makes them more precise and better at denying area and toppling vehicles.
You can’t sprint in any of these games. None of these games have armor abilities. Halo 3 has deployable equipment, but that’s it.
Melee is much weaker in Halo 1, and there’s no auto-lunge, you just swing and hit or miss based on if you were close enough and aiming right. Halo 1 has very few aim assist mechanics. I might as well say Halo 1 is the most difficult and different game of the collection, it’s not the game you should start with.