According to what I've looked up, which is literally just looking things up on Wikipedia if I'll be totally honest, there were two productions of the BAR, the M1918 and the M1922, along with a few improvements/redesigns, the M1918A1 and M1918A2. According to what I looked up, the US army apparently tried to get out conversion kits to swap all BARs to the M1918A2 but couldn't get enough conversion kits out there to get them all. Now, I know just about zero about guns, so I really couldn't tell the difference between them all, so I just chose one at random. I can swap to the M1918 if it really bothers anyone though.