Fallout: New Vegas Project Director Josh Sawyer stopped by the Bethesda forums to discuss firearms and why certain ones might appear in the Fallout world:
Close to it in some cases, especially when it comes to sidearms. The M1911 is still in service in some U.S. military and LEO branches and it's been 99 years since it was introduced. The Hi-Power has been even more successful worldwide, though for "only" for 89 years. The 9mm Luger/Parabellum/NATO round is practically omnipresent in handguns and submachine guns despite being 108 years old. Rifles and machine guns saw the most development in the first half of the 20th century. The difference between a M1903 Springfield and an AR-15 is huge. The difference between an AR-15 and an M4 is not. The U.S. military's assault rifles still tend to fire .223 Rem/5.56 NATO, their marksman/sniper rifles still tend to fire .308 Win/7.62 NATO, and their sidearms tend to fire 9mm NATO or .45 ACP.
The Fallout games are also set in the United States which, practically speaking, means the private collections of citizens are as relevant as what LEO/military groups have. The U.S.'s "curio and relics" collector license system means that when a firearm is 50 years old, it can be transferred through the private market with a C&R license instead of a more restrictive FFL. If the End of the World came tomorrow, the number of private citizens rolling out with AR-15s, M1 Garands, Mauser K98s, AK-47s, Thompsons SMGs, BARs, Hi-Powers, Mosin-Nagants, and M1911s would be pretty large.