The ammo I suspect of being corrosive is one or both of two batches:
- Yellow box, red and black lettering, "CHINA NORTH INDUSTRIES CORPORATION", headstamp C J / 92, and/or...
- Red and black box, gold and white lettering "NORINCO/CHINA SPORTS", same headstamp "C J / 92".
I hear what you're saying wjshark and agree that you can't just let barrels sit around uncleaned. However, I've seen corrosive ammo effects before and this sure looks like it. If the Norinco is corrosive it's not as bad as some nasty old German 8 mm I used to get many years ago.
The climate where I live is not very humid most of the year so you can get away with neglect better than you could in some places. My standard practice for firearms I'm not going to use for a while is to clean thoroughly and annoit liberally with rust inhibiting grease. I also keep silica gel in whatever container they're in.
I've inspected all of my .223 rifles that may have had this ammo fired through it, and they are all OK (because I cleaned them properly).
On edit: I just got through inspecting (in in a few cases cleaning) every firearm in my house, and that is no easy task! Even among low-value ones stored in similar conditions to the rusted AR barrel and in similar state of lax cleaning, the AR barrel was the only one with even a trace of brown rust (and it had plenty). I'm pretty sure that ammo had something to do with it. It may be only mildly corrosive, but that combined with my negligence it kicked my ass.
I promise to clean my weapons better from now on and never put bad ammo through an AR-15 again.