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You cannot put sub-gague inserts into a pump gun / or a semi-auto shotgun.... you can only use them in break open action shotguns ( Over Unders, Side by Sides, single shot guns...)...
... in theory you could drop the chamber maters or whatever into the chamber of a pump gun ...but you sure cannot cycle 20ga shells thru a 12ga magazine reliably...or safely...without all kinds of issues...
The typical 20ga load is 7/8 oz of shot ( or "00" or whatever you want to shoot )....and its easy to find 7/8 oz loads for a 12ga.....just buy a 12ga pump and you'll be fine...( you'll get less recoil with the 12ga gun anyway / than a typical 20ga that is probably a full pound lighter ).
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