It will easily fit into your break open shotgun barrel and accept any 9mm Luger ammunition.  After firing simply remove the adapter and tip upside down to remove the spent 9mm shell.  This adapter has a rifled bore to increase accuracy. This in NOT a smooth bore adapter. This adapter has a rifled bore.  The 9mm cartridge side is precision CNC machined to the exact chamber dimensions to accept 9mm ammunition (SAAMI chamber specifications).