I have never cared for back pocket carry much... It takes away one of the advantages of pocket carry -- being able to casually obtain a firing grip. As a result, I think its a lot slower.
I have also been hesitant to sit on my gun. An R-9 would take it, but... I think sitting on a Kel-Tec P-3AT could be a problem. I am sure people do it, but...
I think the R-9 is much better suited to front pocket or jacket pocket carry.