I had an old timer tell me once that obviously the gun reacts differently when held down by a sled or other device then it does in your shoulder. Yes you could get a gun clamped down where it will not move and zero it in but whe pen you hold it, it kicks, the barrel moves, it's not going to be the same. Trick is to keep your hold etc. the same so the recoil has the same effect each time. That might not be right but it makes sense to me. I've always used sand bag or something to help with steady but want the gun to be in my shoulder and free to move however it's going to move once I pull the trigger.