The chamber cast shows it to be a 20 gauge shotgun with a 2" length. I confirmed this with a brass 2-1/4" shell which wouldn't seat all the way. A 2-1/2" Cheddite would not either, but cut to 2" it seated perfectly. At the throat and just past, the bore is around .615 and the grooves are around .635. I presumed it was made for a .635-.640 ball in a brass case, given the manufacture date. A plastic or paper shell (which I also tried and measured) would not have the ID to hold a ball large enough to fill the barrel. This would be 16mm +. My surmise is that they took a 20 gauge barrel made to the same OD as the 16, then bored and chambered it for a cylinder bore 20 gauge and rifled it to a depth of .010 or so per groove.
When I get some balls close to the right size, I my plan is to upset a ball into the grooves, knock it out and measure it.
I had never seen a setup exactly like it but find it interesting. The result is very close to what I thought I was getting (16/.577) but more interesting. This was only four years after the 577 Snider was superceded by the 577/450, but BSA and others were still making sporting guns in that caliber. I thought maybe Sauer was making these for the German market. I would have thought they would have made a 20 gauge with a reduced bore for ease of loading or the more obvious of simple a 16 bore in both with shallow rifling. Maybe from a manufacturing standpoint this was the best option but it seems like a unique piece.
The pictures in the add don't really describe how pristine the gun is.The barrels blueing looks a little suspect, but the rest looks absolutely original and the barrels litterally like a new gun. There is a small splinter gone off the thin edge of the forearm and a tiny crack on the inside of the forearm behind the wedge that I have superglued and clamped to stabilize, and that's all I can find.
