I don't like conversions. I would just buy a separate .22lr. A conversion can be about the same in cost as a separate .22lr rifle. Also, you can treat the other gun like it's not your AR-15. In other words, you don't have to baby it. Likewise, you don't have to freak out if a friend is not treating the .22lr rifle with the best of care.

Plus, I view an AR-15 as being a self-defense tool. I don't like mucking around with my self-defense tools. I keep it simple. My self-defense tools are ready to perform their duty whenever they're not being cleaned. There's never any thinking about whether they're in the appropriate condition for their intended task.