Tinker,

The chambering question is one of the most puzzling aspects to me also. It seems intuitive that a reamer would clean it so a standard round would fit.

The sling issue is easier. I didn't use a sling at any time with the double. With any of my other rifles I would have used a hasty sling from the sitting position and would have expected accuracy about equal to accuracy off a bench with forearm supported, rifle in the left hand and a hasty sling in use. With the double, on both the offhand and sitting positions, I used a left hand grip near the front of the forearm with fingers part way around the barrels.

Emory