That's a good point, powderkeg, but not always the case.
Simply dumping the powder out onto a piece of white paper and then shining a light into the can will tell you whether it's a rust issue or not.
Also, the two -- rust vs deteoriated powder -- look VERY different. Nitric Oxide (I think that's what it is, but can't remember for sure) is powdery.
Rust generally has a... chunkier... look (for lack of a better term) and is darker.
Which also brings up another very important point...
If the interior of the can IS rusted...
WHY is the interior of the can rusted?
It's because the powder has started to deteoriate (sp?) and has released fumes that are corrosive to the metal.
The ONLY time I have ever seen the interior of a can of powder rust is because the powder rotted.
I have a half dozen cans of powder in my basement right now, all of which were purchased in the early to mid 1980s, and none of them show the slightest indication of rust on the interior of the cans even though two of them are showing surface rust on the exterior, courtesy of my fingerprints and damp basement conditions.
If there is dark to bright red, powdery to chunky contamination of any kind in the powder, chances are good that the powder is not good.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.