The Bushmaster Carbon 15 Flat-top Carbine features lightweight Carbon 15 Composite Receivers that are 40 percent stronger and 40 percent lighter than standard aircraft aluminum components. Carbon 15 is not affected by moisture, cannot corrode and has no surface finish to wear off. The Upper Receiver is fitted with an Anodized Aluminum Picatinny Rail for unlimited optics/sight mounting. The Flat-top's 16-inch barrel is turned in an M4 configuration and chambered for 5.56 mm NATO caliber and accepts .223 Remington equally well.