position the bag far enough forward and to the side so that you have to lean heavily on the bench - this will steady your body.
rest your gun on the bag. if the barrel and slide are long enough to put it on the bag, do that (you'll probably scorch your bag a little). if not, firmly position the bottom of the grip on your bag, and make sure you have as much of your arms on the bench as possible.
when benching a pistol it takes a little more concentration to get everything set up, but you can do it w/ some effort.
i run power pistol and 115 xtp's thru my beretta 92...