system
Given the following post, respond as if you're commenting on Reddit:
human
[Halo] Why are some of the human weapons so crap?

The flamethrower fielded by the UNSC (the usefulness of such a weapon a separate issue) has absolutely pathetic range and weighs an absolute tonne, forcing the player down a great deal. A WW2 era flamethrower had a range of up to 50m, and was nowhere near so heavy.

More surprising is the shotgun, which has somehow been made to have almost no range at all, and is still pump-action only. By the third game has only a 6 round magazine, whereas a modern KSG can hold up to 15.

What made humanity drop the ball so hard and forget how to make a decent urban assault weapon?
gpt
The flamethrower that the Master Chief is recorded as having used in the battle of Voi isn't actually a flamethrower made for combat but an industrial strength mobile incinerator. These were made for mobile decontamination and burning through obstacles, they were intended to be used at close range and be mounted on a heavy lifting exoskeleton. It was only thanks to the Master Chief's enhanced strength and protective armor that he was able to use it as a weapon.

As for the rest of the weapons, a big issue that humanity faced when first encountering the Covenant was that weapons designed for killing other humans had great difficulty penetrating covenant armor, and could rarely penetrate energy shields. Weapons had to be adjusted to fight this new enemy by firing fast enough to overwhelm them or by increasing the power of the weapon. The shotgun is a case of the latter, by increasing the power and size of the rounds it uses it is able to effectively penetrate covenant armor and shields at close range in a fairly devastating way, but it can only hold a few rounds and the recoil is such that it is much slower to fire.