I suspect the auto-advancing harmonica gun concept stood the best chance of occurring before the mid 1800's. Yes, lathes could easily make cylinders, but I bet there'd be trouble trying to make Webley-esque cam grooves in them using the tools of the day. The earliest machineguns were strip fed (or belt fed, a very close sibling). A harmonica would be easy to bore out, as well as to add cam grooves to. Layout would be very STEN-like, most likely (a very unintuitive form, which is probably why it was never thought of). As with any CB gun shooting more than one round, the real trick is preventing/accounting for chainfires.
What has always amazed me is not that the self-loading pistol was never developed for BP, but that recoil or blow-back SMGs never occurred earlier. Heck, JMB used a spoon to turn a lever gun into a machine gun; hard to imagine BP fouling that 'gas trap' before dozens of shots were downrange. Do remember that the notion of reliably shooting more than a dozen or so bullets without malfunction was the fever dream of a madman back in the early revolver days, which is why the Colt/etc. were such marvels. Something like the STGW57 roller delay system (similar to but much simpler than the G3) was likely possible with the tools and materials of the day. But as I said before, these leaps were utterly divorced from both the expectations and extensions of the state of the art, which is why they could not be derived in those eras.
Before repeaters, ammo was manually assembled in the chamber. Then when that function was automated by cartridges, it was manually loaded into the chamber. Eventually this loading was augmented by machine parts to make repeaters, to simplify the manual loading of cartridges from storage to chamber. Early machine guns automated the manual loading motion, semi-auto systems later automating the trigger release for better control. Expecting the leap from cap and ball to semi-auto handgun would be like jumping from revolvers to a fully-integrated heads up aiming display that automatically guides the shooter's hands to the target.
BTW, for the folks saying DA revolvers in a competitive format only arrived with contemporary autoloaders, I would submit the S&W Model 3, circa ~1870. Yeah, it was expensive, but as if any autoloader wouldn't be. But, for the low pressure/thrust, big bore, black powder cartridges available, it was arguably a superior design in many respects to the swing out architecture we enjoy today. It was also a reliable, effective, and war-ready sidearm, arguably the envy of anything offered at the time.
TCB
What has always amazed me is not that the self-loading pistol was never developed for BP, but that recoil or blow-back SMGs never occurred earlier. Heck, JMB used a spoon to turn a lever gun into a machine gun; hard to imagine BP fouling that 'gas trap' before dozens of shots were downrange. Do remember that the notion of reliably shooting more than a dozen or so bullets without malfunction was the fever dream of a madman back in the early revolver days, which is why the Colt/etc. were such marvels. Something like the STGW57 roller delay system (similar to but much simpler than the G3) was likely possible with the tools and materials of the day. But as I said before, these leaps were utterly divorced from both the expectations and extensions of the state of the art, which is why they could not be derived in those eras.
Before repeaters, ammo was manually assembled in the chamber. Then when that function was automated by cartridges, it was manually loaded into the chamber. Eventually this loading was augmented by machine parts to make repeaters, to simplify the manual loading of cartridges from storage to chamber. Early machine guns automated the manual loading motion, semi-auto systems later automating the trigger release for better control. Expecting the leap from cap and ball to semi-auto handgun would be like jumping from revolvers to a fully-integrated heads up aiming display that automatically guides the shooter's hands to the target.
BTW, for the folks saying DA revolvers in a competitive format only arrived with contemporary autoloaders, I would submit the S&W Model 3, circa ~1870. Yeah, it was expensive, but as if any autoloader wouldn't be. But, for the low pressure/thrust, big bore, black powder cartridges available, it was arguably a superior design in many respects to the swing out architecture we enjoy today. It was also a reliable, effective, and war-ready sidearm, arguably the envy of anything offered at the time.
TCB