Alan,
You may want to look back at the thread •Re: Guns lost at Champion Hill and BBB ... since there is quite a bit of discussions of Whitworths in the Western Theater there from years ago and some specifically discussing Raymond and Bearrs' saying Bledsoe had one there that burst. Most think Bearrs was mistaken about that or at least it was not a breach loading one, however, there is a website (I think the Battle of Raymond website) showing a digger that actually found a Whitworth bolt there. Whether or not this necessarily means breachloading Whitworth I am not sure.
Jack