Agreed.
First of all please excuse the crude drawings, they are not to scale and do not represent the actual part geometry. I'm a visual person myself so I thought this would help.
The first picture represents the rifle and the bullet as two solid, independent bodies. Because the bolt is locked until the bullet passes the gas port, they can be analyzed this way. The blue arrow represents the force of combustion acting on the bullet and the rifle body. This action is equal and opposite. We know that the mass of the rifle body is much more than the mass of the bullet. We also know that the force acting on both bodies is equal. If we consider the equation F=ma (force = mass x acceleration) we can conclude the the acceleration of the bullet will be much greater than the acceleration of the rifle body. Furthermore, we can use the muzzle velocity and bullet weight to figure out the energy given to the rifle body (I keep saying body because I want to emphasize that at this point that the entire rifle can be treated as a solid piece).
I'm sure you're all familiar with the second picture, there are much better pictures and animations available. I just wanted a visual aid. After the bullet passes the gas port, the gas is sent to BCG, unlocking the bolt. The rifle can now no longer be treated as a solid piece.
http://img694.imageshack.us/img694/5900/87778343.png