The Browning .50 (M2) is over 80 years old and still in service for the foreseeable future. And the Browning-designed M1911 is approaching a century and still going strong.

The AK follows the British Sten gun concept: design it to be cheap to manufacture and to work "good enough." A brilliant design, but not alone in that league. However, neither of the Browning designs I cited qualify as cheap to manufacture. But they work well enough that both will see a century in service.