The Kalashnikov assualt rifle (AK-47) and WWII

The StG 44 is still used today as well in the Middle East, so that beats the AK-47 with a few years. I wouldn't be surprised if you could find MG 42s in service today too - the Finnish military used it (or at least spare parts from it for the NATO version, showing how close they are) for a long time.

Heavily modified Medium Tank M4s are still in service in South American countries, and I wouldn't rule out the M1 rifle being used in South America completely either.

 

.50 M2 machinegun has been used since 30's. And M1911 pistol has been used since, well 1911...