Here's my take on this, I started all this so I'll at least weigh in.
I think Sarah's shotgun is a hybrid prop, a SPAS-12 with a wooden pump action grip added to it. James Cameron had many of his "futuristic" weapons culled from other parts, the Aliens weapons are a good example of this. Even some of the vehicles and planes built by his production designers were taken from conceptual drawings earlier drawn by Jim. The Sulaco ship for example was basically the shape of a Machine gun, not sure which exactly. The APC was a converted British Airways plane tug, the famous drop ship was a Vietnam era Chopper fitted with other Brit jet parts....
Anyway I'm guessing this shotgun was a hybrid.