450s were nerfed near the end of year one to make them weaker in PvP. They reduced their range and long range aim assist. I'm also fairly certain they nerfed the damage too. With the release of forsaken, they retuned all weapons to favor crits over body shots and autos were really the only weapon type that didn't benefit from this. Since their damage didn't increase, but the health on enemies did...they would just get weaker over time. So it's not necessarily that autos kill too slow on their own, it's just that the other weapon types perform much more quickly and flawlessly at their respective range. Autos kind of just fell out of usefulness once everything else had become so much better.