A great point. One of my SKSes was not caked in cosmo, but the firing pin was completely seized (and unfortunately seized in the protruding position) to the point where I had to use pliers to take it out AFTER I took out the retaining pin. It was covered in this black stuff with a molasses type of consistency. I am glad I checked that before putting rounds through it otherwise it would have been slamfire heaven. Turns out that the trigger sear also seized after one subsequent shot (after the firing pin and channel were cleaned thoroughly). Turns out that the sear was seized from the same gunk, causing an inability for the trigger to reset.
Strangely the rest of the gun did not have any signs of cosmo or anything else. Regardless, lesson learned is always check the firing pin in particular and make sure that the bolt assembly rattles when shaken.