AR-15 is more akin to our M-4. Modified stock, shorter barrel, shortened recoil and adjustable features. There are other differences as well. I don’t know if it is the most popular weapon, I’m not a huge fan of the platform.
They still make AR-15's with full length 20" barrels. The platform served me well in the Marine Corps and I own two of them plus the AR shotgun - MKA 1919.