Had my 590A1 for a while now. It came with the standard 18.5 inch barrel. Really like the gun but I wanted a bit more utility from it. I've read that the barrel from the 825 ulti-mag fits right on. Mossberg had been out of stock for a couple of years now... at least every time I checked, local stores never had any in stock, Gunbroker stuff was new price for used barrels and over retail when a new one would show up. Recently Mossberg had what I was looking for back in stock. I used my "Christmas gift" from my wife to get one and find out for sure. The barrel indeed does mount right up. The only part that isn't interchangeable are the chokes. The 590A1 and I suspect the 590 as well use the same chokes as the 500. The 835 chokes may be unique to that platform.
FWIW the barrel from the 500 also mounts up but you need to swap out the magazine tube.
For those still reading...Part of my motivation for doing this was I didn't want to show up to shoot trap and/or skeet with my Citori725 and have everyone think how much I suck at it with such a nice shotgun.
On;y done those once before although I've done sporting clays a number of times. Suck at that too
FWIW the barrel from the 500 also mounts up but you need to swap out the magazine tube.
For those still reading...Part of my motivation for doing this was I didn't want to show up to shoot trap and/or skeet with my Citori725 and have everyone think how much I suck at it with such a nice shotgun.
On;y done those once before although I've done sporting clays a number of times. Suck at that too 
