Some of the guys around here, had .32 S&W inserts for their .30/06's and .300 Magnums, back in the 70's and 80's.
I've never considered an insert for a shotgun.
I do have a LONG .22 Hornet insert for a 16 bore sitting on the rack. Don't even remember where I got that from. I don't have a 16 bore now, maybe I should get one so I can use up all this .22 Hornet ammo I have?

It might not be safe shooting it in an insert, as the ammo was for a CZ or modern .22 Hornet only.