An Automatic Rifle, firing pistol cartridges?
I'm not sure where you got your information from but it was always intended as a Submachinegun, something light and with enough firepower as well as high fire rate to clear a trench, hence it's first nickname, the "trench broom" during WW1 but it came too late to the party.
Read your own article, its original design concept was as an automatic rifle but the issues with the breech made .45 ACP a necessity as opposed to a full calibre rifle cartridge. A shotgun was also considered a "trench broom" so that designation doesn't necessarily imply SMG or whatever you were going for there.