According to Stephen Moffat, the gun that River is using is the same one that used to belong to Jack. He didn't exactly have it on him when he 'left' the TARDIS, so all of his things from that time are still in there somewhere. At some point, she dug around a bit and found it. As for the digital rewind, maybe she didn't know it was a feature of the gun, since it wasn't hers first.