Googled which weapons cops have out of curiousity and the AR-15 isn't in the top 10 according to this list:
https://www.criminaljusticedegreehub.com/popular-guns-for-la...
https://www.criminaljusticedegreehub.com/popular-guns-for-la...
The end of that list shows the M4 carbine - which is an AR-15.
In popular vernacular, an “M4” is a shortened version of the “M16”, which is the military version of the “AR-15”. Armalite originated the design and Colt licensed it.
There’s a bit more to it than that, but that’s the high points.