-The BR-55 was delayed for twenty years because it initially fired a smaller bullet in hyperburst. At the UNSC's request, Misriah to scale the caliber up so it was effective against shields and Elite/Brute physiology. Unfortunately, they couldn't get hyperburst to work with a 9.5mm bullet, and other moved goalposts resulted in the gun getting shelved for over a decade.

-Speaking of the battle rifle, the pistol grip doesn't actually look like that.

-Spartan Lasers were introduced sometime near the end of the UNSC-Covenant war, as was GUNGNIR armor.

-Most of the armor we saw in Halo Reach is canon. Some of the armor in Halo 3 is canon. None of the armor in Halo 4 is canon.
>HAYUBASA is canon, sort of. It was the 2549 equivalent of 600 dollar toilet seats, and was shut down before a prototype was fabricated. Nobody is quite sure how it got funding in the first place.

-There's a lot of functions of MJOLNIR, ODST armor, and common weapons that we never see.
>The bullet counter on the MA5 series doubles as an electronic scope.
>Misriah hasn't shipped an unguided missile for the M41 Surface-to-Surface MAV/AW since the weapon was introduced.
>Magazines for the SRS99 larger than four rounds exist.
>If you look under the dash of the Warthog, you'll find a bullet hopper and a magazine reloader.

-The UNSC are not idiots.
>They have never fielded mechs. There's been a few forklift-type exoskeletons, but those are few and far between and reviled by their operators.
>Yeah, there is an artillery vehicle called the Cobra. No, it doesn't transform like that.
>Also, the Rhino family of vehicles has seen a tremendous number of guns, but no plasma cannons.
>There's no way they'd do something as stupid as A)Build a three-story Winnebago with no seats for the operators B)Mount a MAC to the roof that's so low-powered, it has to be driven to within several kilometers of the target C)Design the cannon so that it can only be targetted by a handheld laser, and not have a crate of those lasers in the cabin.
>The Vulture is a 2500s version of the AC-130 gunship. If represented accurately in Halo Wars, its missile barrage and autocannons would be the equivalent of off-map artillery support.
>Booster Frames were a popular toy licensed by Mattel-Tonka. Sales really took off when the anime "Baku Baku no Mecha" was translated and syndicated for multi-planet broadcasting.

-Neither are the Covenant.
>Elites can be pragmatic and honorable. They can be pragmatic and dishonorable.
>Elites may try to do the honorable thing, but combat weeds out the idiots with brutal Darwinism. A Zealot isn't going to enter into a swordfight if he isn't sure he'll win. And there is not a shipmaster who would leave his shields down because he wants a good fight.
>Yeah, they DO know how to recharge their weapons. And Halo: CE is the only accurate representation of how many shots each gun holds.
>The plasma pistol lacks a scope. As such, training for Grunts and Jackals focuses a lot on point shooting, with the pistol's fire control refining their aim. So yes, autoaim is canon. =D

-Halo 4/Spartan Ops are AU.

I have a few more ideas that are essentially 'fixes', like the electronic scopes on the MA5s. But first, sleep...