You’re in for a surprise if you ever look in a patrol car. Every agency I know of, even in a very rural state, has an AR as standard equipment - either in the cabin, or in rare cases, in a trunk lid mount.
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.