AR does not stand for Assault Rifle in AR-15.

It still is one though, a lightweight semi-automatic rifle which is classified as an assault rifle. Being as it was the model created as a prototype for the US military in 1956 to 1959. Which was later turned into the M16 with very little modification in 1964 it entered military service.

The term "modern sports rifle" didn't exist until 2009 and until that point all of these models were refereed to as assault rifles. Changing the dictionary, doesn't change what it is. It simply changes what you can legally get away with, under the NRA, you know that corrupted by foreign influence organization.