There are two main kind of smgs ingame: 9mm (Germany and Britain) and .45 (US and US lend and lease). 9mm kills with three upper body shots, .45 with 2. .45 loses power faster though and has higher recoil.
edit: There is also 7.62 Mauser in the Sauer38H, which IIRC is the only bullet that doesn't always 1s1k with a headshot
The same is the case for Ppsh41 and PPS43 bullets in the beta afaik.