I certainly wouldn't call the Mauser pistols cheap or simple, although they were at least mass-produced, sort of. Ever handle one or shoot one? One of my first guns was a Mauser C-96 purchased around 1966. It came with a shoulder stock/holster (I was living overseas at the time). They were a marvel of design and manufacture. That doesn't mean they would have been beyond the technology of the 1860s by any means. That was only 30 years earlier, after all. But there would have been two things.
The first is obvious. It hadn't been designed yet, just in the same way the Browning Automatic Rifle hadn't been designed in 1896. But by 1896 the other problem had been overcome: the ammunition.
Between the end of the Civil War and 1896 (our newly established base year for discussion), smokeless powder and jacketed bullets had been successfully introduced, both of which largely solved the ammunition problem. Ammunition wasn't perfect yet (is it perfect now?) but good enough for automatic weapons. Still, submachine guns didn't show up for another 20 years and even then, they did not become common for yet another 20-plus years. Some successful designs were introduced before the war to be sure, including the Thompson, but one couldn't say they were common (or cheap or simple).
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.