It looks like I answered my own question. I went on a grouse hunt last evening and this morning (3 flushes, 6 birds total flushed) and afterwards decided to try my French 16 boxlock with slugs. I figured the barrels are nitro proved, so I decided to take a chance. Here are the results from 50 & 75 yds with my ic/choked right barrel, using normal 1 oz. non-saboted Federal slug loads:
50yds off hand resulted in the shot on the left, while the second shot at 75 yds using a tree rest, holding a couple inches high, resulted in the near bullseye. To say I was surprised a smoothbore sxs barrel with a standard brass bead would be anywhere near this accurate is an understatement. Looks like I'm good to go out to 80 yds or so on a deer, which is about the range I'm most confronted with in a bird hunting situation.
