I would buy a used .556 AR-15 (there are thousands hitting the market). Build it the way you want and sight it in to shoot accurately at 400 yards. Then, buy a Ciener .22 conversion kit (they are the best in my opinion...I have one and it is flawless). They drop right in and since your weapon is already sighted for .556 at 400 yards it makes it a perfect plinker at 100 yards. The kit costs $149.00 right from the manufacturer. I have shot thousands of rounds of .223, .556, and .22 through mine and have loved every minute of it.
Here's a link: