Tactical Life has published photos of SIG Saur’s not-yet-named upcoming submachine gun.
It obviously looks similar to the AR-15, but it is unlikely many parts are interchangeable other than the pistol grip and the trigger group.
According to SigSaurGuns.com it will be available chambered in 9x19mm, .40 S&W and .357 Sig.
For years people said the submachine gun was dead. I don’t think it ever died. There will always be a niche where sharing a common cartridge between both primary and secondary weapon will be an advantage.
[ Many thanks to Aaron for the tip. ]