I forgot to post this yesterday, but I found this video really interesting from a historical perspective as well as a gaming perspective.
Mark Felton: WW2 Guns in Modern Service
Out of all of them, the idea that the PPSh-41 is still out there is amusing to me. Out of all of them, the idea that the M1911 Colt and M2 HB .50 cal and the DshK are still in use isn't surprising at all. It's weird to even think of them as WWII guns, because WWII kind of came along during their run, just like so many other wars did.
Still, there is something interesting about PCs running into really old guns in current, regular-military service. Don't put your copies of GURPS World War II aside, you can mine them for firearms for your 2021 GURPS Action game.