Windham Weaponry's RMCS-4 kit offers four calibers in the same AR platform by simply changing out barrels, magazine wells, and if necessary, the bolt/bolt carrier and magazine. The 9mm barrel has a melonite finish, and the other barrels are chrome-lined for long wear and easy cleaning - machined from 4150 chrome moly vanadium steel and finished in black manganese phosphate. The receivers are CNC machined from forged 7075 T6 aircraft aluminum with a black hardcoat anodized finished, and the bolts are machined from Carpenter 158 steel.