— Item examination - Japanese
"It shortens the total length of the shotgun while it extends the barrel."
— Item examination - unofficial translation
The Shotgun Parts (ショットガンパーツ shottogan-pātsu ) is an item which was intended to appear in Resident Evil 0 during its GameCube stage of development but was removed in the final release. You can get it in the Biohazard 0 TRIAL EDITION with the help of cheat devices such as Action Replay MAX or GameShark; however, it is not usable and lacks a 3d model as well.
It is very likely that this item was supposed to be combined with the Shotgun.