A receiver for a firearm is provided with anchoring structures for mounting an accessory part having complimentary anchoring structures. At least one anchoring structure on the receiver comprises a through hole and a complimentary through hole on the accessory part such that when it is mounted on the receiver the through holes are aligned such that a fastening pin can be inserted. At least one anchoring structure comprises a portion of the lower edge of the receiver that engages a complimentary anchoring structure on the accessory part. At least one anchoring structure comprises a first projection and an adjacent first indentation at the upper, rear portion of the receiver with said first indentation being further away from the through hole than said first projection. A complimentary anchoring structure on the accessory part attaches in the manner of a snap mechanism.