This Shotgun is used in Terminator 3. It looks similar to a Remington 870 with a military bayonet attachment. But the shotgun in the pictures has a unique sling attachment, an odd forearm, no bayonet lug and the heat shield looks like a factory Mossbergs.
Here is a picture of a military Remington 870 with bayonet attachment, for comparison sake.
Here is a picture of a military Remington 870 with bayonet attachment, for comparison sake.