These aluminum adapters will allow for the use of any .410 round in your 12ga. Shotgun.
They are made to chamber in a 2 3/4" gun (so they work fine in a 3" chamber) and are made to support the full length of a fired 3" .410 round. You may need to poke the fired .410 casing from the adapter. Easiest way to do this is with another .410 round.
Manufactured by the Ordnance Group LLC in Florida, USA.