The shotgun folks routinely have 410 bore and 28 gauge tubes they stick inside their 12 gauges. Some are on really expensive guns and apparently work really well.

Know people have put 22 rimfire liners or subcaliber chamber inserts in rifles. But has anyone actually put a full power smaller caliber liner in a big bore ? Something like a 223 Remington barrel inside a 458 Win Mag or maybe a 30/06.

Hammer