From The Manufacturer/Wikipedia/People Smarter Than Us:
When your job is to serve and protect and your life is on the line, count on the Smith & Wesson M&P-15, the latest standard in reliability. Each model is chambered in 5.56mm NATO/.223 Remington and offers a 6-position telescopic stock with dual sling attachments, dual-position front sling swivels, a MIL-spec flash suppressor compensator, 4140 steel barrel, a 7075 T6 aluminum receiver, and a hard-coat black anodized receiver finish to prevent wear.