From some interview videos, here is one tip. I thought I was going to try it, but I always forgot: use a string to tie a brick hanging on the front the barrel (make sure it's not a very light one
), and hold this gun aiming target for some time, a few minutes to a few hours every day, practicing this for a year or two to control hand shaking. The logic was this: one day when you remove that brick, the gun is feather light, your arm is strong enough and there will be no shaking when shooting off hand. Same can be applied to rifle, two hands in rifle case, of course.
God knows