I actually thought it was carbon buildup at first...until I couldn't remove it. Since it's in a place that's not supposed to have it and Midway was having a sale (plus free shipping) I fixed the problem. But...I'm the only person I know IRL who cleans weapons to armorer-turn-in level after every trip to the range.
Man I hate cleaning weapons. I could just never really get a feel for it no matter how hard I tried. That's one thing I like about shooting my longbow - wax the string now and again, replace the rubber bands on the strings (to quiet the twang) when they get worn, and un-string the bow when you are done shooting/hunting for the day.
I just disassemble mine and take them to work. I dump the parts into the ultrasonic cleaner next to all the jewelry. Take them home, lube everything up, and reassemble. Voila.
When I was in Army basic training after one particularly long, muddy, firing lots of blanks field exercise one of my drill sergeants had us take our weapons apart and take the barrel/upper receiver portion into the shower to (I guess) wash the major portion of the dirt/carbon off. In the shower with a gun....but no cookie! Side-note I don't know why he was video taping us....maybe he was making a training video or something! I mean that makes sense, right?
We usually make our own cocktail. But, truth be told, we've been trying a pre-made cleaner that's working well enough and it lasts. With four bench jewelers, we usually have to change our chemicals every day. But, we're getting a full week out of Polychem's Ultra-CR.