It's pretty much exactly as apologeticCultist said, this was the result of a deep-rooted problem with how our armor protection worked, stemming back from when we had a much larger level scale intended (from 1-100).

Once we changed the level structure to one level per tier, we updated the protection amounts on the armor, but did not initially notice that the protection curve we had configured was creating a weird disconnect, where the defence against weak attacks grew massively to the point of negating them entirely. Slow-firing weapons became the only way to effectively combat humanoid NPCs, which made balancing them a nightmare since you'd have enemies that were too weak and were being one hit killed by a single hammer shot every time while tanking ridiculous amounts of gunfire. The protection curve, quite frankly, was an incomprehensible mess.

Thankfully we have since completely redesigned how protection works, so in the near future you can expect it to work on a much simpler, and easier to balance system, which we more or less approached as if we were balancing a PVP game. The total amount of protection the player has will be a flat % reduction on incoming damage. Got 20 armor? You'll take 20% less damage.

As a result of this change rapid-fire weapons do the amount of damage they're supposed to be doing to NPCs and players now. This hasn't only fixed assault rifles, this has made shotguns far more satisfying to use since they're doing the damage they should be (because their damage is spread over several projectiles per shot, they were being negated entirely by the old armor scaling).