"A custom part for the Matilda that allows the gun to fire 3 rounds per pull of the trigger."
The Gun Stock (Matilda) is a custom part in the Resident Evil 2 remake.
It allows the Matilda to fire 3 bullets if the fire button is held, it can still shoot one bullet per button press if the button is simply tapped.
Inside the trunk of a police car at the Parking Garage, it requires the Car Key in order to open it. The key is at the outside part of the Firing Range, on a table inside the Tin Storage Box and next to the Equipment Disposal Notice.