AK47 rifle
The AK – introduced in 1947 – is the most successful design in firearms history.
It was a Russian design whose intrinsic reliability was far, far better than the M16's one.
Despite being heavier, it practically never jammed and used thirty rounds magazines instead of the M16's twenty rounds one.
The AK47 shoots a heavy and slow bullet that gives it a shorter range and a trajectory more difficult to master for it's user, but whose power can be considered on par to the M16 thanks to it's larger diameter.
If the American rifle went under several changes over the years (up to change it's whole name to 'M4') the AK is a good rifle even for nowadays standards, and it's exactly the same that was used during the Vietnam War.
The only real update it ever received – during the fifties already – was starting to build it using a lighter alloy.
Very few weapons has ever been so long-lived in firearms history.