<Tr> <Th> Caliber </Th> <Td> 12 gauge, 20 gauge, and . 410 bore </Td> </Tr> <Tr> <Th> Action </Th> <Td> pump action </Td> </Tr> <Tr> <Th> Muzzle velocity </Th> <Td> 1,325 ft / s (404 m / s) for 12 - gauge, 2 ​ ⁄ ", 00 buckshot load 1,560 ft / s (480 m / s) for 12 - gauge 437 - grain rifled slug </Td> </Tr> <Tr> <Th> Effective firing range </Th> <Td> 130 ft (40 m) </Td> </Tr>

What does the a mean in mossberg 500a