"Defensive .22LR" is an oxymoron. Sell the M&P15-22 and get something that works. Or just get something that works like an M&P9.

Headshots? You're kidding, right? The head is extremely hard to penetrate, even the Pope took a "headshot" and survived. The area around the bridge of the nose is about the only place you'll get penetration. If you can hit it. Target of last resort, but great gun store talk...

-- Chuck