Interesting question
It's quite difficult to pick just one innovation. The evolution of firearms is like a house of cards, each innovation supports the next level of invention. Rmove one card and the hole thing tumbles to the ground.
That said, it I were to pick one revolutionary concept, it would be self-contained cartridges. The introduction of pre-loaded, self-primed ammunition made the gun something that was simpler to use, safer, more effective and much more reliable.
Although even as I type this, I can think of a dozen other innovations that could be #1.