Kel-Tec RDB 5.56mm NATO 20in Blued Semi Automatic Modern Sporting Rifle - 10+1 Rounds - RDB stands for Rifle Downward-ejecting Bullpup and the 17 points to the barrel length. This variant is the original and features a high-impact resistant Zytel forend with built-in Picatinny-style rail on the bottom for mounting accessories. Like all the RDB variants, it's completely ambidextrous, accurate and compact.