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.Specifications: CALIBER: .223Rem WEIGHT UNLOADED: 7.1lbs MAGAZINE CAPACITY: 20 or any AR15 Magazine OVERALL LENGTH: 30.6" BARREL LENGTH: 20" TWIST RATE: 1:7" TRIGGER PULL: 4lbs