I have seen the ads for "multicaliber adapters" for rifles. Some old-timers in the Adirondacks used to keep one in their pocket for shooting a grouse or snowshoe rabbit with a head shot using their deer rifles, but with a smaller and less meat-destroying cartridge. They worked pretty well for close-range shots from what I was told. The adapters came in such calibers as .22LR in .222 Rem, .22LR in .223, .30 Carbine in .30-30, .32 ACP in 30-30, .32 ACP in .30/06, etc., etc. so you are shooting a full bore-sized projectile down the barrel. You just opened the action, took out the "standard" cartridge, slipped the loaded insert in the barrel, closed the action, and shot your critter.
What these are is a rifle cartridge size case with an opening at the base for a smaller cartridge to fit in. (In the rimfire "smaller cartridge" models, there is also an insert with an offset firing pin to allow firing.)
The last ad I saw recently they were priced at $20 for blued steel, or $28 for stainless. The producer was MCA SPORTS/ACE BULLET CO., 2800 West 33rd Road, Anchorage, AK 99517-2201.