.69 pistol? Black powder?
Yeah. And smoothbore. Which basically makes it a 6 inch 14g shotgun. Does frightening things at close range.
Highly unlikely.
I know when people make a rifle like an AR-15 or custom benchrest shooting rifle, they start with a factory receiver that’s been serial numbered and registered, then they make/buy and install the barrell, stock, trigger, etc. themselves. But it always starts with a factory receiver, that seems to be the smallest piece that is legally considered a gun.
For the OP:
A friend of the family (sort of an honorary grandfather) made his own shotgun out of a couple of standard plumbing pipes. You take a narrow pipe that just fits a shotgun shell, insert it into a larger pipe that just fits the narrower pipe and then weld a BB onto a cap on the bottom of the larger tube (as a firing pin). He even went a little overboard and put a camera mount on the cap so that it would look like a monopod for taking pictures, but you could turn it around and start shooting. Somewhere out there is a very silly video of him and some friends testing it and simulating the covert assassination of South American dictators (well, cardboard cutouts of said dictators) using their camera-pole/pipe-gun.
In California, circa 1990, he was not required to register it and it was not illegal. However, laws change and I suspect that’s no longer true. For example, he had legally owned a WWII-era 20 mm anti-tank gun; changes in the laws around '95 meant he had to turn that over to the authorities because it was no longer legal to own.
It may not be the whole state. Where I live (an unincorporated part of the county), they also ask us to dial 911 even for non-emergencies. A mile up the road (which is within a city boundary), that’s not true anymore.