I would say the sidelock was a slow evolution, the revolution was the boxlock. A LOT of things had to happen, to allow either design to progress to where it was, circa 1900. Few of those things happened all at once. Or, in one place. I don't think any single development, ie, the self contained cartridge, or theLefaucheux break action, taken alone could be held up as the single thing that allowed sporting firearms design to change to what we now know it as.

But, the boxlock comes close.

Best,
Ted