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