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Correct. Not the only engine type that used High-pressure blank cartridges that were modified from firearms cartridges either. If you're already making a type of cartridge for firearms anyway, it's very simple to use the same design for other uses (like engine starts, without having to re-invent, or re-design anything or build a new factory.

https://image.sportsmansguide.com/adimgs/l/1/105820_ts.jpg
https://global.discourse-cdn.com/business6/uploads/cartridgecollectors/original/3X/7/5/7576f44287df7446d88263df8768d78d4bd30932.jpeg
(The original starter cartridges were all-brass, like the top pic. The plastic ones came later, as they were cheaper to make and buy, and there were still engines in use that required them.)

https://www.youtube.com/watch?v=IACjOvyx5hs

There were a few companies that made shell starters. They were usually found in military applications, especially aircraft rotaries and engines for heavy armor. IIRC, only a few used actual 12ga cases while the rest were just similar in shape.

@BulletStopper

Concrete drivers (.22s/.22LR) and early ground radar i.e. Jurassic Park (12ga) come to mind. The latter have mostly been replaced with multi-frequency devices.

I think that you are confusing one thing with another. This was an interim technology, a design that came before the advent of powerful batteries and electric starter motors.

The first thing he did was insert a starter cartridge into the mechanism. That was to provide the gas pressure the get the pistons moving. The second thing was to provide an initial ignition source, to provide for fuel ignition during start-up.

The last thing he did was to tap the firing pin for the starter cartridge with his hammer to fire it and start the pistons moving. At that point the engine starts and the whole thing becomes self-sustaining.