590 has plastic trigger guard, 590A1 has thicker barrel and aluminum trigger guard. My 590 has bayonet lug (M9 bayonet) came with a heat shield (590A1 comes with no heat shield because of the front ghost ring). All 500 Receivers are pretty much the same, so in theory if you know what your doing and can find the right parts, you can turn a 500 into a 590....I have a 590...but it's got a Black Aces Quad Rail, green laser and a strobe flashlight both with micro switches, flip up iron ghost ring sights, red aluminum mag follower with higher pressure spring, MagPul stock with Limbsaver Recoil Pad, single point sling Much easier on my shoulder now...but it's heavy. The added weight to the front of the gun does help with recoil. I can shoot 3" or 2 3/4" I usually have 9 2 3/4" 1oz deer slugs loaded. Spread pattern on a 590 is horrible even with 00 buck.