Maybe an earlier Winter War, say 1931-34, or so. The Russkies get the shit smacked out of them somehow (earlier Stalinist purges?). Then, by a stroke (haha!) of luck, Stalin has a minor stroke, is out of action for a while, and a triumvirate or something rules in his stead. They see the sense in a change of doctrine, appoint better military leaders, and start changing the hardware. Maybe the '31-'34 war showed that soldiers start engaging in much closer rangers, making the bolt-action rifles totally crap, and they decide to mass-produce fully-automatic small arms, starting with a submachine gun in early 1936. They continue development, and in late 1937, they make a variation chambered for more compact rifle rounds, rather than full-rifle rounds. Stalin recovers by mid-1938.
When the second World War rolls around, as it inevitably would, the Soviets are much better prepared, and Stalin's purges haven't hacked away at the Soviet military of the late 1930s.
Bye-bye Barbarossa, hello Nevsky.