Your options (in no particular order):
0) Get a Ruger 10/22 and tacticalize it. There is a HUGE aftermarket for these rifles, and you can build yourself a rifle that fits your own unique tastes and/or mission (the base 10/22 platform is a mature and reliable product). RimfireCentral forums even has a section dedicated to the Tactical 10/22. LOTS of experience and good info there.
1) Get a purpose-built AR-platform rifle, such as the S&W M&P15-22 or the Colt Umarex. These rifles have polymer plastic receivers and hand guards, and while they can accept a limited number of big AR parts, they will never be anything other than a .22 rifle. Accuracy and reliability still hit or miss.
2) If you already own an AR-15, get a dedicated .22LR upper that can replace your centerfire upper. These seem to be pretty pricey, but the benefit is that you can use milspec AR-15 parts for everything except the BCG.
3) Buy a complete AR-15 purpose-built as a .22LR rifle. This is probably the most expensive way to go.
I have a M&P15-22 because I wanted a semi-auto rifle and was enamored with the AR platform (I also own two AR-15s). If I had it to do over again, I'd probably get a 10/22, and not necessarily tacticalize it, but would definitely spend some time/money making it as accurate and reliable as possible.