Got a new barrel from a maker that starts with a K. Love it and it's very accurate. So last fall I put it up without cleaning since I had just shot it twice to check the zero. Over the winter it stayed in my dry bedroom and a injury made me [\put off cleaning it for a few months. Much to my chagrin when I looked down the bore there was a huge glob of red rust about 4 inches from the muzzzle. Much cleaning and now I have a barrel with about 100 test rounds down it with a nice pit and part of a land missing. It looked like you had put sulfuric acid down it in that little spot and left it for 5 months. I've let my sako/tikka ss barrels go years without cleaning and no issues at all. What can I do to stop rust in my SS ar barrels?? Just have to rush home and scrub 'em and slather them with grease? Is there a treatment for SS barrles that stops rust?

stainless will rust, especially if they are contaminated with iron during manufacturing
pickling/passivation with nitric acid is used to remove the cross contamination
you can by a paste to do it yourself if needed

Be very careful that pickling paste is no joke.

Stainless steel is stain-LESS not stain free.  It will rust and the way it rusts can be worse then carbon steel rust.
In order to prevent rust you shouldn't just fire the rifle and then put it aside and not expect it to rust.
The old timers who said "Never let the sun set on an uncleaned gun" were right then and still largely right today.
You CAN get away with storing an uncleaned and unprotected gun, but then you will get caught short once in a while.

As above, there could be something "off" about the barrel steel.  I'd contact the maker and ask them what the Hell!.

To prevent rusting in a gun barrel all that's really needed is to do at least a partial clean to remove carbon and powder fouling, then run a patch with a few drops of CLP Breakfree through.
CLP is an excellent rust preventing product and I've never seen rust on a gun that had an intact layer of CLP on it.

There are plenty of other products, some people just leave a film of Hoppe's #9 bore solvent in the barrel.
This prevents rust and works on removing any copper or carbon fouling.

Also, much depends on where and how you store the gun.  Put it in a damp place like a closet or basement and there's a good chance it'll rust.

If you have a situation where it's simply impossible to do even a quick clean and application of as rust preventing lube, if nothing else spray a good shot of a spray lube or even WD-40 down the bore and store it muzzle down so the excess will drain.
Almost ANYTHING is better then nothing.

416 Martensitic Stainless Steel Bar carbon content is 0.09 min/0.15 max %.

As you found out, it rusts without protection.

You don't have to be crazy cleaning to be in a good habit of cleaning. Just remember, dirty and oily is good, dirty and dry is bad. What I recommend is taking a bore snake with you to the range and just snake it with some clp before and after leaving. Put a couple of drops into the gas ports of the BCG, rack it a few times, and you're good to go. That should and ought to afford you way more than enough time to clean it when you want/need to. Nothing really difficult or time consuming.

You shouldn't be following the trend of "not cleaning" as many on the forum seem to do. You don't have to completely clean the gun each and every time, but you can put oil in it when you cannot break it down and clean it as thoroughly. Any sort of oil will do, even motor oil.

I just wonder if you somehow left some sort of cleaning agent in the barrel and forgot?

I would be wary of exposing high-strength steels (particularly the barrel extension) to acids, or anything that produces nascent hydrogen.  I would be more cognizant about cleaning and oiling in tje future.

Try cleaning it with a brass cleaning brush, and then shooting it.

What ammo were you shooting?
I ask because the only ammo I've had rust with is Bear ammo. My SS mini 14 rusted out after shooting brown bear and my M1A rusted out after shooting silver bear.

I got a brand new rifle I never shot from a known manufacturer that starts with a P. Never shot the rifle because the bolt had pry marks.  I inspected the uncoated SS barrel and there was rust all over the extension. I live in the desert mind you, with a pretty good storage area that is no way exposed. Mind you, I've had the rifle for only 15 days. I could see this happening if there's no CLP after several thousands or hundred of rounds with a SS barrel, but right when received? Not right.

I have some nitrided barrels that have never been shot and some never cleaned, a few nitrided ss barrels I've never oiled that are sitting and never built and they don't have any rust at all.

Say Again...?

Should have proofread to repel GD scum, lol

Is that barrel extension missing a lug, at 1:30?

Yes. It's the type of AR.

I don't think that is rust. It looks like some sort of packing grease.

It’s stain LESS, not stain FREE.

Expose it to a salt, moisture, and air long enough and it will rust.  Just takes longer than other materials.  Particularly if you scratch it.

Stainless steel forms a sacrificial chrome oxide layer on it’s surface.  That is what provides the corrosion protection.  Scratch through that and subject the substrate to salt and water and it will rust.

Is this a joke?

It was rust. Took me a good minute to remove it.

Just because something "took me a good minute to remove" does not mean it is rust. Looking at your picture, the "rust" is too uniform and straight to be rust. Your "rust" is also on the aluminium receiver a little, if I look at it really closely. It's just some sort of packing grease that you just noticed under bright light is all.

As far as OP, the "rust" is most likely copper or something else. Depending on lighting conditions, it might be better or worse looking. Rust does not occur uniformly on surfaces.

Or using cheap steel jacketed ammo.

If have time to shoot you have the 5 minutes to do a very basic cleaning. Run a couple wet patches down the bore. Spray the bolt group and upper receiver with cleaner, wipe with rag, then douse with oil. Run wet bore brush down the barrel a few times. Then a wet patch and a couple dry ones... repeat that a few more times, then a patch wet with oil. Put it back together. There, no rust.

The residue of shooting is highly acidic.   Also, take a smell of your suppressor after shooting.  You will never leave an uncleaned bbl. again.

Elmer Keith once said, "The sun never sets on a dirty gun."  Wise advice.