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What you’re describing is a “reset trigger,” more or less, and they are indeed legal.
You could also just make a crank operated gun. Those are legal, too. The key is you have to keep turning the crank to keep firing. Your continued action is necessary.
Forced reset triggers are illegal per ATF.
There are cranks that you can add to an AR-15 pattern rifle to make them into a Gatling type operation. I think some are “universal”, but are designed to work on the AR.