ASIDE FROM SOME INTERNAL DIFFERENCES THE 590 IS EXACTLY WHAT ONE PREVIOUS REPLY INDICATED. SIMPLY A 500 JACKED-UP AND ON STEROIDS. HIGHER CAPACITY, BAYONET LUG (ON WHICH WILL FIT ANY M-16/AR-15 BAYONET) AND AN ASSORTMENT OF VERSIONS ARE ALSO OTHER DIFFERENCES. GHOST RING SIGHTS, SPEED-FEED STOCKS ETC. THE 590 PASSED ALL THE REQUIRMENTS OF THE MILITARY AND BLEW-OUT ALL OTHER ENTRIES. HAVING OWNED ONE AND USED IT EXTENSIVELY I CAN STATE THAT IT IS ONE TOUGH PIECE.