This reminds me of back in my guiding days. I would far rather the hunter have a 30-06 or 270 he could really shoot than a magnum he was afraid of and could barely keep all shots on a hillside. Or shoot a 30" triangular 3 shot group on an elk at 40 yards with a 300 wby with no misses. Yeah, it can be done if the shots hit the right places.
As Taylor mentioned the dovetails may be too deep to make a safe 20 bore. I have seen photos of a split in the bottom of a dovetail of a TC Hawken or one of the clones it was 54 IIRC. Dovetails, even shallow ones, are major stress risers and any barrel that has seen much use (or even no use in some cases) will usually show some "anomoly" in the bore at the dovetails unless its a really heavy barrel. A tight patch on a short jag can be very informative....
Dan