I'll be illustrating with a few stock photos from Mossberg. First the 500. The 590. My beloved favorite, the 590A1. As Shown: The first, the 500 Persauder, Blued and synthetic, 20" barrel, 8 round capacity. The 590 Special Purpose, Matte finish and synthetic, 20" barrel, 9 round capacity. The 590A1, Parkerized finish and synthetic, 20" barrel, 9 round capacity. The extra round is accomplished by a redesigned magazine tube and cap, allowing an extra round of 2 3/4". Both the 500 Special Purpose and 590 Special Purpose are available either blued or with a matte black finish. But the magazine cap and bayonet lug quickly distinguish the one from the other. The differences between the 590 Special Purpose and the 590A1 are a little more difficult to spot immediately. Both have the redesigned mag and bayonet lug, but the 590A1 features some changes specifically to meet military standard 3443. The changes made to meet the military's stringent standards include a heavy wall barrel, and metal (aluminum alloy) trigger gaurd and safety button. The 590A1 receiver is also stamped on the left side with "M590A1". While any of the three can be configured nearly identically with the many options and part #'s available from Mossberg like rear ghost ring and front AR styled sights, adjustable or Speed Feed stock, cruiser pistol grip models and heat shields, the 590A1 is the only of the three to feature the heavy wall barrel, and metal trigger gaurd and safety button. Additionally, there are a few part #'s in the 590A1 family available only to military and LEO's, such as the 14" barrel "Compact" model with a 6 round capacity, and 13" LOP, and the 14" barrel "Compact Cruiser", 6 round pistol grip model. I own both the 500 Persauder/Cruiser 20", and the 590A1 20". Brad.