I have not mic'd the bores. I need to get batteries for my electric calipers if I get to town, they were dead when I got it out. The rounds drop into the chambers of both barrels with ease and show good accuracy with the KKM once I get them in the chamber.
I am hoping the different shape of the Buffalo Bore HC loads might make a difference. Midway did not have the Double Tap when I ordered or I'd have tried those too. I agree about the HC heavy bullets out penetrating the Extreme Penetrators, that's why I haven't tried them before. I saw the same in the tests I've been able to find. I don't want to go to them unless I have to.
Researching Buffalo Bore, Double Tap, and Underwood before starting this I saw a lot of positives for Underwood. That plus them being cheaper and me planning to shoot a lot of them started me in that direction. I might end up having to try a different brand.
We will see when my orders show up. Glad this thread is helping some people.