I'm cheap and I shoot alot, so I bought a "22 conversion kit" for my AR-15 Assault rifle and I love it!
Converting a assault rifle into a 22 is a great way to practice at up to 100 yards:
AR-15 with a 22 bolt and magazine
Smaller bullets are way cheaper, and you can save cash using smaller 22 bullets:
Smaller bullets are 10x cheaper
This shows how easy it is to change out a Colt A-15 .223 bolt worth a .22 Long rifle bolt, giving you 10x the target time for the same money.
It costs 40 cents for a 2.23 bullet vs. only 4 cents for a 22. LR bullet, a 10x cheaper ammo.
See here how easy it is to convert your AR into a .22 caliber semi-automatic rifle: