Depends on which brand of shotgun your talking about.
Out of the box the Maurshin M500 Series shotguns are all metal and don't require any other metal parts.
The Maurzen CA 870/ M870/1100 series are plastic and you can get metal bodies and metal parts for the CA870 and M870.
As far as I know there isn't a metal body for the Mauri M3 series.
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