+1 to the shooting .22s. Being young and broke, all I really shoot is .22, but it sure has helped my marksmanship.
My friend was getting very frustrated because he couldn't barely hit a 5 gallon bucket at 50 yards with his Mosin Nagant standing. I gave him my Marlin .22 and told him he should get in the habit of putting the gun on target with his body, not his muscles. After emptying my magazine, he went back to his gun, and knocked it over first shot.
If you want something that will really piss you off enough to work on your offhand all day, try 50ft smallbore 4 position. If you twitch, you're in 0 land. Haha.