I’m afraid it’s back to weaponry.
I was thinking about how rifling is a fairly inelegant solution to the problem of maintaining the accuracy of a bullet’s flight. Today’s invention is therefore an alternative mechanism to promote greater accuracy.
A round is injected into the (unrifled) barrel of an automatic weapon -but the whole barrel is spinning (powered either by an onboard motor or by the exhaust gases from a previous round).
The round seals in the barrel and spins with it. A firing pin acts in the usual way and the propellant drives the bullet down the spinning bore -expelling it with an optimal rotation rate determined by the distance to the target, the windspeed, the cartridge size, etc.