I really don't think Firearms are going to change much in a way that the layperson will really understand in 50 years. We already have weapons that are capable of high rates of fire and are accurate. These are rifles that are using technology developed in the 1950s.

I think the major advancement in firearms technology that will be absolutely game-changing but nobody but firearms owners will notice are smoother triggers for striker-fired pistols and for bullpup rifles. Other than that we are just going to see better manufacturing practices and more modular Firearms then we are already seeing. We are also going to see more standardization of firearms. In America I think we're going to see more rifles that are going to borrow the ergonomics and interface of the AR (like the Ruger Percesion Rifle).