Which World War II weapons could still serve today?

Which World War II weapon systems could still be useful in modern service?

Bofors guns and Browning small arms remain plausible, while an M5 Stuart would require extensive modernization rather than continued service in its wartime form.

Several gun and small-arms designs remain plausible candidates, including tank guns, the Bofors 40 mm, the .50-calibre machine gun, the M1911 and the Browning Hi-Power. These examples could still perform a useful task. The Brazilian Stuart example involves a different proposition: substantial modernization of an old tank. The tank is first called an M3 and then corrected to an M5 during the exchange. An upgraded vehicle therefore needs to be distinguished from the question's stricter requirement to use a wartime weapon unchanged.