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The Chiappa Firearms X-Caliber 12ga/9mm Gauge Adapter Insert is designed for use with 9mmx19 ammunition. While, yes, you could shoot a 380 through a 9mm barrel the headspacing is different. 380 is 2mm shorter and will have to make a "jump" those extra 2mm which will cause wear and gas cutting. Overtime it will cause more damage and not allow a standard 9mmx19 to seat properly causing dangerous pressure spikes.
The Chiappa Firearms X-Caliber 12ga/9mm Gauge Adapter Insert is compatible with +P and +P+ ammunition in light amounts. Heavy use of +P or +P+ will cause excessive wear and degradation, shortening its service life.
The manufacturer does not list any restrictions on chamber length. I am not aware of anyone using epoxy to secure the adapter, since that could cause permanent damage to the gun if you ever removed the adapter, but I would image the extra stability wouldn't hurt accuracy.
Yes, this can handle +P ammo, however there have been reports that +P groupings are more erratic than standard 9mm.