NOT having much input from anyone, I pulled the trigger on a side charge upper+BCG from Bear Creek.
I've wanted to try a side charger as well as single shot, so I did. Plus the ejection port is enlarged to accommodate the larger rounds.
Probably this weekend I'll order the .458 bolt. All that's left (all, ha) is the barrel & fore end. Gas block and tube if I want semi-auto function.
I'll put this on an existing lower, until I sell my National Match Ar.