There's nothing here yet
There are many who feel that the 5.56 NATO is a superlative rifle round. Much has been said about larger alternatives to 5.56, such as various 6.5mm and 6.8mm rounds among others. Less has been said about smaller rounds. Off the top of my head, I can recall that there was a German 4.6x36mm round, used in the HK36, and the British 4.85x49mm round. Neither of these rounds managed to gain widespread acceptance. My knowledge of the voodoo that is ballistics is somewhat limited, so I'm uncertain as to whether these failures were caused by flaws with the rounds themselves, or because they were below some lower limit of effective bullet size, beyond which performance decreases rapidly. Could we see a resurgence of these concepts in the future, or do they represent an evolutionary dead-end?