Early model BARs did have semi auto. This was dropped on later models.
Technically, the BAR is classed as a LMG (light machine gun). There are several later designs of LMG that are slightly heaver, and still feed of a 20 (or sometimes 30) rnd box magazine. Some of them have quick change barrels, but not all of them do.
Remember the BAR was basically the first, lightest automatic rifle, and while it goes 18lbs or so, its lighter than a Lewis gun. The BAR was built for the concept of "walking fire", to give fire support to the doughboys walking through no-mans land. There was even a metal belt attachment to hold the butt of the gun.
While this tactic did not work out as well as was hoped, the BAR proved very useful, even if it was not as well suited for sustained fire as a belt fed gun.
Browning virtually gave the govt the design (he accepted their first (low ball) bid, under the condition that the first of the new rifles were sent to his son's unit in France.
All else being equal (and it almost never is) bigger bullets tend to work better.