To unlock the Sawed-Off Shotgun in Call of Duty: WW2 you have to level up the expeditionary division all the way to the end of level 4. The way that you level up the expeditionary division is to use it in one of your classes during matches. Then enter the headquarters and go see the division prestige guy:
Select the expeditionary division and proceed with the prestige. This unlocks the Sawed-Off Shotgun no matter what your soldier level is.