I have shot 870's for 32 years and am very familiar with the,
My mental illness is such that I remember trivia, while forgetting important stuff like things at work
I recall a close-up of M. Douglas' hand when he pops the safety on the gun, the shape of the guard and the saftey looked like an 870 to me.
The gun in the movie appeared to have a shortened butt stock, not just a pistol grip. It also had a cut down barrel that I thought still had a vent rib on it.
There was a seen where he was shown stuffing shells into the gun's mag, about 1/2 a box. You only saw the gun from mid-fore end back in this seen. I am sure, off camera, you would have seen the mag cap was removed and the shells were falling out the end of the mag tube.
It was a waste of an 870 Wingmaster