Why a .22LR Conversion Kit On An AR15 Rifle:
Using either a dedicated .22LR upper receiver or a .22LR conversion kit is a great way to teach a new AR shooter the fundamentals of shooting the rifle without the intimidating loud noise of the full size battle ammunition, and for the experienced shooter to maintain proficiency at a fraction of the cost of shooting full size .223/5.56 ammunition.
To shoot .22LR ammunition out of an AR platform, you basically have two options. You can buy a dedicated upper receiver/barrel set, or buy a .22LR conversion kit that simply replaces your bolt carrier and fires the .22LR bullets through your AR barrel. The dedicated upper’s barrel will usually be a 1 in 12″ twist (as opposed to your AR’s 1 in 7″ or 1 in 9″ twist). The 1 in 12″ twist will result in a better stabilized .22LR bullet in flight which may result in slightly better accuracy. .22LR rounds fired from a conversion kit and through your AR’s barrel may or may not be as accurate as those fired out of a dedicated upper, but I have been very satisfied with the results using the simple and less expensive conversion kit. I can easily hit coke cans at 50 yards, and a full size paper plate at 100 yards. This is good enough for training purposes.
I live and swear by my CMMG brand .22LR conversion kit. When properly maintained, it is both reliable and accurate. They are around $160 and usually come with a 25 round magazine.