Mossberg 590 A1 rust?

Your pictures are on Photobucket and they have a policy that prevents them to be hotlinked to unless you pay.

Download them onto your computer and the attach them as files here.

Yeah Photobucket is a sham.

So what's the question here? All firearms can rust. The 590 has a pretty good park job on it but it isn't immune. A good scrubbing and light coat of oil should fix it.

For the chamber I would use steel wool on a cleaning rod spinning with a hand held drill. For the rust inside the trigger group I suspect you may have to take it apart to clean or replace any rusted or damaged parts. When done just wipe down with your favorite oil/lube and check on it now and again. The chamber "appears" to be a little rough, would that have given that area a place for rust to start forming?

Have you soaked it? Also I had a similar issue on my 870, lots of wd-40 and elbow grease made it go away. I tried several oils, but wd40 got it done for me.

Have you kept a round in the chamber for a while? I did that once, came back after a number of years abroad, and the steel casing (only looks like brass) got stuck to the chamber. Meaning, that might be rust from the shell you had in there, and not the chamber itself rusting. Look at the shell that was in there and see if the "brass" was rusting.... Best,
Romulus

Meant to add, go ahead with the advice you've received to remove the stain in the chamber, it all sounds good to me. My technique is to pool some Flitz on, let sit, then wipe...it will work I promise you, and it will save your park. Might take a few sessions, but it will work

Once you've taken away the rust, get a medium oil such as normal 30 wt. Motor oil and slather all parkerized surfaces with it. Let it sit wet all night and wipe clean. What parkerization is is like a fine sponge, it absorbs oil. This is where the corrosion resistance comes from. It holds oil better than the others.

Also the M590A1 is a true warsman's shotgun. It is without question the toughest and best pump action military shotgun in human history.

I just got a 590 A1

I agree with Cooldil on treating your parkerization with oil, I prefer breakfree though. A good park will absorb the oil if you slather it on.