"Parts such as the fore grip and barrel upgrade the weapon to a SPAS12."
The Shotgun Parts is an upgrade item that can be found in Dino Crisis.
These parts can be combined with the Franchi PA3 to increase the firepower and transform it on Franchi SPAS-12.
In the locked storage, at the Stabilizer Experiment Room. The password to unlock it is "1281".
Like all the other upgrade items, when playing with Regina's prehistoric costume, the parts will have a different appearance.