Clean mine the way I was taught and never had a problem. No dental mirror required.
Was it a POW, or did you have a 2 shop?
I remember some soldiers who claimed that the best way to clean it was to put on a blank adapter, then fill up the barrel and upper receiver with break free, let it rest for a minute, then fire a couple of blanks through it. Nothing quite like high pressure solvent to rip out carbon.