The SAR 21 ("Singapore Assault Rifle - 21st Century") is a bullpup assault rifle designed and manufactured in Singapore, chambered for the 5.56x45mm cartridge.  This variant utilizes standard AR-15 magazines.  This weapon came with both the quad-rail barrel group and the original barrel group with integrated scope and laser. 

Click Here for a video from Forgotten Weapons about the SAR-21.