Dogbite, I wouldn't worry about the "gun too powerful" argument. It was famously (infamously?) used against Harold Fish in Arizona, costing him a huge amount of money and a big chunk out of his life. However, most of us feel that with proper material and expert witness testimony, that argument would have been neutralized when he was tried. GT's search function will bring up more details on the Harold Fish case if you are interested.
In a pawn shop -- likely to be full of customers the one time you don't want it to be -- overpenetration would be my biggest concern. Many 10mm loads tend to penetrate deeply. Best bet, I suspect, would be something on the order of the 135 grain/1450 fps load offered by Cor-Bon and also, I believe, by Double-Tap.