<Table> <Tr> <Th_colspan="2"> SAR 21 </Th> </Tr> <Tr> <Td_colspan="2"> The SAR 21 with the attached Round Corner Firing (RCF) module </Td> </Tr> <Tr> <Th> Type </Th> <Td> Bullpup assault rifle </Td> </Tr> <Tr> <Th> Place of origin </Th> <Td> Singapore </Td> </Tr> <Tr> <Th_colspan="2"> Service history </Th> </Tr> <Tr> <Th> In service </Th> <Td> 1999--present </Td> </Tr> <Tr> <Th> Used by </Th> <Td> See Users </Td> </Tr> <Tr> <Th_colspan="2"> Production history </Th> </Tr> <Tr> <Th> Designer </Th> <Td> Tuck Wah Chee and Felix Tsai of Chartered Industries of Singapore (CIS, now ST Kinetics) </Td> </Tr> <Tr> <Th> Designed </Th> <Td> </Td> </Tr> <Tr> <Th> Manufacturer </Th> <Td> CIS: 1999--2000 ST Kinetics: 2000--present </Td> </Tr> <Tr> <Th> Produced </Th> <Td> 1999--present </Td> </Tr> <Tr> <Th> Variants </Th> <Td> See variants </Td> </Tr> <Tr> <Th_colspan="2"> Specifications </Th> </Tr> <Tr> <Th> Weight </Th> <Td> SAR 21: 4 kg (8.82 lb) SAR 21 GL / M203: 5.3 kg (12 lb) SAR 21 P - Rail: 3.6 kg (7.9 lb) SAR 21 MMS: 3.5 kg (7.7 lb) SAR 21 Light Weight Carbine 3 kg (6.6 lb) </Td> </Tr> <Tr> <Th> Length </Th> <Td> 805 mm (31.7 in) (SAR 21, SAR 21 40 GL / M203, SAR 21 P - Rail) 680 mm (26.8 in) (SAR 21 MMS) 640 mm (25.2 in) (SAR 21 Light Weight Carbine) </Td> </Tr> <Tr> <Th> Barrel length </Th> <Td> 508 mm (20.0 in) </Td> </Tr> <Tr> <Td_colspan="2"> </Td> </Tr> <Tr> <Th> Cartridge </Th> <Td> 5.56 × 45mm NATO </Td> </Tr> <Tr> <Th> Caliber </Th> <Td> 5.56 mm (0.22 in) </Td> </Tr> <Tr> <Th> Barrels </Th> <Td> Single barrel 1: 12" and 1: 7" twist options </Td> </Tr> <Tr> <Th> Action </Th> <Td> Gas - operated, rotating bolt </Td> </Tr> <Tr> <Th> Rate of fire </Th> <Td> 450--650 rounds / min </Td> </Tr> <Tr> <Th> Muzzle velocity </Th> <Td> 970 m / s (3,182 ft / s) (M193) 945 m / s (3,100.4 ft / s) (SS109) </Td> </Tr> <Tr> <Th> Effective firing range </Th> <Td> 460 m (M193) 800 m (SS109) </Td> </Tr> <Tr> <Th> Feed system </Th> <Td> 30 - round detachable box magazine; proprietary or STANAG magazines </Td> </Tr> <Tr> <Th> Sights </Th> <Td> 1.5 x or 3x optical sight; back - up iron sights </Td> </Tr> </Table> <Tr> <Th_colspan="2"> SAR 21 </Th> </Tr>

What is the back up sight used in sar 21 in the event of optical scope failure