It is a Colt 1878 12 ga., somehow "sleeved" on the outside of the barrels to make it look like an 8 ga. They used 12 ga. blanks in it when it was shot, but Mortensen(sp?) carried brass 8 ga. shells on his belt.

There was originally just supposed to be a few token shots of it in the movie, but as Mortensen got into the character more he lobbied the director to let him carry it all the time, kinda grew on him.

May God bless America and those who defend her.