The Logic Of Weapon Evolution In Lore

You had effective weaponry on both sides in Halo Reach. Then in CE you went further (with the Covenant losing a few good ones sadly). Alright. After that, you went downhill. Going from a powerful Magnum that definitely made it something from the future to a peashooter in Halo 2, to something worse in Halo 3.

the magnum dominated in halo multiplayer and singleplayer. They changed it to try to balance the sandbox. at the time of halo 1 updating a game, especially on console, did not really exist. so the attempts at balancing weapons took the form of trying to do stuff different in the next game. now we are in the age of updating code being very easy and done frequently.

now if this is being said lore wise, well the weapon differences I think are described as it being different models of weapons being used across different UNSC forces and also different gear coming into use and out during the war. halo reach show cased the UNSC navy, marines and army. Halo 1 was all UNSC navy, the ship was stocked originally for a special spartan mission and had some non standard equipment/ upgrades on the ship. halo 2 was earth UNSC navy mostly, but obviously happening later in the war and reach (a hub for the military) was lost. showing how earth had to rearm afterwards in a live action trailer as factories and more were brought online to begin manufacturing more gear. Halo 3 they were scraping the bottom of the barrel in weapons, vehicles, and other gear.

You’ve got the order wrong. Reach was the last Bungie game that came out. The order was CE, H2, H3, ODST, followed by H: R. Then started 343’s games with H4 then H5: G.

I believe the canon reason for the difference between the pistols, as @rootbeerener had in-part mentioned, was that the Halo CE pistol is a modification of the officer’s model of the base pistol, while the rest are standard sidearms. In essence, all these pistols are M6, with most being magnums which fire somewhat explosive rounds. The M6D is a larger, scoped variant that is the one from CE, the others are just normal or lackluster pistols.

Reach is a prequel to CE.

It doesn’t matter when it was in the Halo timeline, the thing that matters is the order of release. Judging a game that came out in 2001 to a game that came out in 2010 aren’t really comparable in terms of weapons added or removed