When I create a pistol grip I use a cylinder with a lot of sides, around 20. Then using the top view select the 3 vertices on the right side and scale them or snap them flat, then do the same on the other side. Then grab those 6 vertices (3 each side) and scale them in toward the centre. Then grab the 6 vertices with the 4 next to them, and scale them in again. Do it smoothly and you end up with a rounded yet chunky grip. Either scale or extruded it vertically and pull the vertices around each side to get them to fit the contours of your referance piciture. When you get to the bottom, extruded a slight amount and scale the face inwards to give a rounded bottom to the grip.