Back in a past life when I helped instruct rifle marksmanship (3 position shooting) we always preached dry firing. I use to spend hours clicking my rifle at the outlet across my living room from prone. My hand on the fore end of the rifle would near turn purple from sling tension.
It paid off the first time I shot KD. I shot rifleman with a 230 of 250. That was with irons on my M1A. There was a bit more back story but it was in northern Wisconsin and we had to trudge through like 18 inches of snow to set and check targets. 400 yards is a long way in full winter gear.