If your definition of a successful firearms design is based on years of active service, then by far and away the winning pattern is destined to be the M1 carbine. This handy little firearm - which we would probably refer to as a personal defense weapon today - was originally conceived as and easier-to-shoot alternative to the Colt 1911 - for use in the hands of non-infantry people like cooks, truck drivers, mortar men, etc. It was developing no time by commercial interests and went on to punch WAY above its weight AND very quickly became adopted as a frontline weapon; something it was never intended to be.
The Amerikanskis produced over 6 million of these darn little things, and they made a big impact in WW2, the Korean War and, yes, Vietnam. As a matter of fact, they were heavily used by the Israelis after that - and they still seem to really like this gun and its cartridge.
And yes these things are so darn good that they still make them today
More to the point, there is clear video evidence to suggest that the M1 carbine will be with us, as an important weapon for several thousand years to come! Anybody who's ever seen the original Planet of the Apes understands that. Charlton Heston's character is pretty good with one of these, as are those nasty horse-mounted gorilla types.
Clearly, no goofy, old school Garands are still in use thousands of years in the future.