It's not just the fact that the bi-pod is on the rail. It's the pressure imparted to the barrel via the bi-pod and rail. Unlikely that the pressure will be equal from shot to shot.
Bi-pods are sometimes "loaded" by putting forward and downward pressure on the rifle. This varies from shot to shot. Sometimes bi-pods are not loaded, which of course means the pressure on the barrel would be different.
Try it and see if you can make the point of impact change. Take off the bi-pod and shoot the rifle from no rest (off hand), soft rest, and hard rest. Then add the bi-pod and try various scenarios.
Last edited by rljatl; 05-14-10 at 21:00