if we’re talking heavy expensive smgs the suomi kp/-31 is good one i will be taking no further questions

Explanation: Amongst the Allies in WW2, both the Soviets and Americans had expensive-to-manufacture submachineguns for close-range combat. For the Soviets, it was the PPD; for the Americans, it was the Thompson Submachinegun.

The Brits made an ultra-cheap ultra-optimized steel tube that could inexplicably still fire rounds, despite being manufactured with only 5 man-hours of work per gun.

Eventually, both the Soviets and Americans would follow suit in seeking a ‘good enough’ SMG instead, with the Soviets adopting the PPSH-41, and the Americans the M3 Grease Gun.