Despite being a nice idea, the "y-39" O think is too advanced for the era.
Well, as i said, the concept is "what a WWII P90" would look like?
So if it's not at least a bullpup (let drop the top-mounted magazine ) it's nonsense.
To you this layout is to advanced for the area?
The concept was first used in bolt action rifles such as the Thorneycroft carbine of 1901, Faucon ‘Balanced’ Rifle in 1908 and in 1936 a bullpup machine pistol was patented by the Frenchman Henri Delacre.
The only restraint was it's unconventional look, which prevented rather concervative military's from adopting it..
To me there's no technological issue.
My concern is to make it look manufactured in the late 30's................