How can i apply physics impulses/set a part's velocity relative to itself or another part?

i'm trying to make a gun that ejects bullet casings out of its ejection port to the right, but every method i've found regarding setting/changing velocities is relative to the world so the casing always flies to the "east" so when i move the casing starts going in the wrong direction.

the methods i've tried so far are:

casing:ApplyImpulse()
casing:ApplyImpulseAtPosition()
casing.AssemblyLinearVelocity =