Abstract: A barrel adaptor for a single-shot break-action firearm comprises a barrel stub and an extractor. The barrel stub comprises a main body and a mounting lug. A barrel-receiving bore extends through the main body and can threadedly receive a barrel. Opposed parallel extractor channels are formed between the barrel-receiving bore and the locking bolt receptacle, substantially parallel to the barrel-receiving bore. An extractor head recess at the breech end of the main body extends into the barrel-receiving bore and into the lug, merging with the extractor channels to form an extractor receptacle. The extractor comprises an extractor head having an arcuate cartridge-engaging lip and opposed, spaced apart substantially parallel extractor arms received in the extractor channels. The extractor is longitudinally movable between a firing configuration where the extractor head is in the extractor head recess and an extended configuration where the extractor head is withdrawn from the extractor head recess.