I think one thing missed from the conversation so far is it isn't just the AK-47 that's kept going this long. The M-16 was first developed in 1964, and it took until then not because of any tech developments between the end of WW2 and the 60s, but because that's how long it took the US military to resolve its internal debate about the best caliber for a select fire rifle. A debate that the Soviets ended up agreeing with when they adapted the AK47 to also fire a reduced round.
Plenty of other nations before and since then developed and deployed other assault rifles. Its sort of been claimed a few times in this thread that there's no point to developing new rifles but that isn't true - there's been plenty of efforts and at different time these have won contracts by various militaries, but the results have been rifles that aren't really any different in performance to already existing rifles. They might be a little better suited to the specific needs of that specific military, but on the whole they haven't raised their infantry to a level beyond others in a way that a new, transformative weapon would.
As plenty have noted the technologies used in rifles just haven't changed that much in a long time. The last time we saw massive changes in rifles was over WW2, and even then it wasn't because of new tech development but because of a changing understanding of the combat environment. At the start of the war it was assumed fighting would be largely static with infantry forces first engaging from 500m or more, and so modern armies wanted rifles that were very accurate at longer ranges. There were actually significant pre-war efforts to upgrade standard rifles, but these efforts focused on developing semi-auto rifles that still fired a large round (only the US got such a weapon in to general deployment). What changed weapon design was when modern warfare was observed, war was mobile and battle lines fluid, with many meeting engagements with first contact at 200m or less. And with more organic support weapons like machine guns and mortars, and more mobile heavier support weapons at ranges beyond 200 or 300m rifles were mostly used for supporting fire, while the bigger weapons did work at those longer ranges. Soldiers just didn't gain much from having a weapon that was more accurate than their enemy's at 500m. What was needed was a weapon that was good enough to suppress the enemy at those longer ranges, but capable of much greater firepower at closer ranges.
So weapons developed during the war that fired a reduced round with select fire, the Stg 44 and M3 Carbine (only the former was conceived as a general issue weapon, though). Shortly after the war the AK-47 was developed, and while its a common misconception that it was copied from the Stg 44 (mechanically they're quite different), it is important to note that there's really nothing going in the AK-47 that wouldn't have been achievable during or even before the war. It just wasn't done because weapon designers weren't aware of how warfare had changed.
There's also something of a misconception in the thread that nothing has changed since then. Warfare has changed again, the assumption that contact ranges tended to be under 300m, and outside of that heavier support would be called in stopped being true in Afghanistan. The nature of hunting insurgents or having convoys ambushed meant support wasn't always available, and the terrain meant contact was often at much longer ranges. The assumptions that led to modern assault rifles weren't quite as absolute, and this was especially true for the latest iteration of the M-16, the M4 carbine, which had traded range for reduced size and weight. But like everything in war, adaptation happened, and a range of rifles with greater power at longer ranges were deployed by all militaries serving in the NATO force. Mostly just adaptations of existing rifles, they were given to designated marksman so infantry units maintained the ability to engage at longer ranges. This isn't advancing technology, but just adapting weaponry to the changes of the battlefield.
Really, WW1 is the last time we saw people conceiving of rifles and not yet having the technology to fully realise the concept. In that war you saw nations realising that infantry units needed automatic rifles for walking fire and to generally boost the firepower of infantry on the offiensive. While the concept was well understood at the outbreak of the war, it was an extreme test of existing developers to produce a weapon capable of reliably performing in the role. A reliable version of the Chauchat wasn't available until very late in the war, which was also the same time the US was able to deploy the BAR. There was a similar story for sub-machine guns - the first weapons were conceived of years before the war, with the Austrians deploying one in 1912 or 1913. But it wasn't until 1918 that the Germans deployed the first machine pistol that didn't completely suck, and it was just after the war the Americans deployed the Thompson as an SMG that was good enough to actually hang around and be used for some time after.
Sorry for the ramble at the end there, the point I was trying to make was around WW1 you could actually look at the improvements in manufacturing and design and see how that will improve existing designs and make new design concepts feasible. But that isn't true now, and hasn't been true for more than 70 years. Now changes in design are more about adapting to changing battlefields, tweaking the trade offs between range,
ROF and weapon weight to suit a specific environment.
Automatically Appended Next Post: 
Totalwar1402 wrote:I just think it’s a dangerous thing to downplay the importance of technology in war and lose perspective on how dramatically things can change in a short space of time.
It isn't about downplaying technology, its about recognising that sometimes there isn't a newfangled doo-dad just around the corner. Sometimes there is a weapon that happens to be the best option for a role and that doesn't change for a long period of time. The was a period of around 200 years that the flintlock rifle was the dominant weapon of war, and over that period it changed and improved only a little. And when it was replaced it was by caplock rifles, made possible by chemical developments, that were more reliable but didn't really change the potency of the soldier all that much. Then you wait another few generations before you get metallurgy and machining improvements to make breech loading viable, and you get the next real change.
At this point we don't even know what the next real change will be, but of all the stuff that's been tried (bull pup, electrically fired ammunition etc) it either doesn't add anything or brings enough drawbacks to be unable to replace existing weapons.
