There's a great deal about the BAR published, starting with Rock in a Hard Place by Ballou. Good book. The dates vary but it started at the end of WW1 seeing a bit of service with the U.S. Army's 79th Infantry Division, which took them into action for the first time on 13 September 1918. I have pics of it in the armory repair racks in the late '60s in USAF possession. Actual combat use existed beyond that of course, but not with the US I think. There's some stories about it being used in early RVN by U.S. forces...I believe that. Lots of older stuff went.
I'm hoping we get some more people jumping in on this particular spot...