I tried that last night. It was a shotgun chamber, thoroughly cleaned and lightly oiled. I do have half a pound of Cerrosafe sitting around, but I doubt that would do a single 12 gauge chamber, and I wanted to do two or three.
Pushed a shot cup type wad down just past the forcing cone. With a 3" clamp, clamped the barrel's retaining ring to the benchtop, nice and vertical. Encircled the breech end with masking tape. Mixed plaster. Poured. After about 10 minutes, the unpoured plaster was hard, so I figured it was time to get the chamber cast out of that chamber. Light tap. Nothing. Firm tap from muzzle end with 1/2" dowel. Still in there. Fairly hard rap. Nope. Trim that dowel just a couple inches longer than the barrel. Get the end inside the shot cup, and bring the whole assembly down hard. Here she comes. Got it out, almost dropped it. Sat it aside. Yes, there is a ring of adhering plaster in the bore.
First things first: WD-40 to displace all the water I poured in there (if 15 minutes in the rain ruins a shotgun barrel, that would be good news for ducks, I think), brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, patch. Still a ring of plaster in there.
Brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush.
A little concentrated attention with the edge of the 1/2" dowel on a stubborn spot. Brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, brush, patch. Looks good through the bore scope. Coat of Break Free, sit it aside. Run over and get the cast. Sand off the part where it overfilled above the case head. Does it fit in the chamber? Hell no. Some kinds of plaster actually come out a little bigger than the hole they were cast in. My can of Dunham's "Rock Hard" Water Putty say it does, and I think it's gypsum based too. This stuff was tight in the chamber because it was trying to be bigger than the chamber.
Conclusion: dental stone is not a satisfactory chamber casting medium.