A year or two ago I almost bought a set of 24 ga. barrels made for the Beretta 686 series. According to the shop that was selling them they were made by Beretta to fit the 20 ga. action. Probably more common in Europe than on this side of the pond.
Here's a 24 ga. 686. It's been sold:
https://www.gunsamerica.com/998139103/60...-shells-NIB.htm