The other day I found 35 .223 cases at the rifle bay, and took them home clean up and add to my range brass stockpile.  I noticed they had some sort of sticky, gummy residue on them that attracted dirt and dust.  Anyways, I figure a trip through my dry tumbler would remedy this, and after a couple of hours, I had shiny brass - except where the residue was.  I was able to spray some Rem Oil on a rag, and wipe the substance off.  It wouldn't wipe off without using a solvent.  It reminded me of what's left after you remove a piece of old electrical tape.

My question is:  Does anyone have an idea what this residue could be from?  Could it be from a weapons lubricant or treatment of some sort?  Most of it was accumulated above the shoulder, or in streaks along the case body.

Brass appeared to be once-fired.  It's WCC 14 with the NATO cross headstamp if that helps.

The mineral spirits or whatever is in Rem Oil took the gunk off.  The brass looked like it was once-fired, however I wouldn't dismiss the possibility they were reloads.  I've left lube on cases after re-sizing before, however it wiped off with a cloth or paper towel.  This stuff wouldn't give up that easily.

If I run into any more of it, I'll pay more attention  before processing it.

I talked to a guy at range once who competes with a AR. I'm not up on the specific classes but his rifle looked almost normal but internally was full of lead to make it weigh more.

Anyway, he showed me some lube that he and a lot of the other competitors use that is a mix of things. One of the ingredients was "STP" maybe your range pick up had some of this stuff on it or something similar.