alright heres what you want to do
1. get your references
side, front, rear and top are what you usually need.
2. start with the side view and model all of the topology of the side view.
make sure you get every curve and do not skip any parts
3. separate the parts
separate all the parts that move independently or have complex geometry.
4. add a mirror modifier
extract the plane to the thickness shown in the front view
then click clipping and delete the faces inside the mesh
5. adjust the loops to fit the shape of the gun
6. detail all the parts