zukifile said:
Here's an inexpensive rest.
Yes, but that's a shooting rest, not a machine rest. The shooter still has to have his hands on the rifle and press the trigger and all that will affect point of impact to some degree. A machine rest takes all human contact out of the picture. Even the trigger is depressed through a mechanical or hydraulic linkage of some kind.
Years ago in his book, Pistolsmithing, the late George Nonte drew a useful distinction between "mechanical accuracy" and " practical accuracy". Mechanical accuracy work is about addressing the collection of factors that make a gun mechanically repeatable from one shot to the next, always locking up the same way, always chambering rounds in the same position, always hitting the primer with the same force, always returning the barrel to the same position with respect to the rest of the gun. Practical accuracy work is about addressing the collection of things that make it easier for the shooter to operate the gun to hit the target where he wants to. Trigger work, putting on better sights, customizing the grips, adjusting the sights for the shooter's vision, etc. Mechanical factors vs. human factors. The machine rest is intended to assess the mechanical factors exclusively. Conventional rests from sandbags to the Caldwell Lead Sled all assess a mix of the mechanical and human influences. They are supposed to allow the shooter to eliminate hold wobble, but they never get there 100%. Imagine some poor fellow with Parkinson's syndrome trying to keep a gun completely still on one of those rests, much less shoot offhand. He could still operate a machine rest accurately, though.
Bart B. said:
If the bullet strikes 2" right and 4" low…
I usually make an initial adjust to match the first shot's error, assuming it looks "good", but after that I use Jim Owens's method of just adjusting only half-way to correction on additional shots, assuming they also look "good". That neatly complies with Nyquist's criteria to prevent oscillation (chasing the spotter) by limiting gain to <1 and incrementally approaches true zero (for your eyes and hold).
I learned this by screwing myself up one time. My first offhand shot at an SR target was a 7:00 full 9. It was an NMC event, so no sighters. I should have made the full adjustment but thought it looked like the shot had let go when the gun was wandering low and left. So I left the sights alone and got a second 9 in about the same place. So then I made the half adjustment but on my third shot really did let one go a little low and left and had a third 9, close to scratching 10, but not making it. Then I woke up and set the sight to put the first hole in the middle and proceded to finish with 10's and X's. So I was shooting well enough that day that I could have had a 99 instead of a 97 if I'd had my wits about me after the initial shot. We all have to learn somehow, I suppose.