Mystery Rifle

Well, I kinda goofed with my post scheduling yesterday – the StG45 IPSC video was supposed to be for today. I’m on the road for a couple days, so in lieu of a regular Monday post, here’s an interesting photo of a rifle we don’t have any information on:

Unidentified prototype rifle
Unidentified prototype rifle (click to enlarge)

I believe the photo is from Aberdeen Proving Grounds, dating in the 1920s or 1930s. Looks like a BAR magazine, and a modified bolt-action receiver turned into a semiauto. Any thoughts?