Was that with the FN? That was the approved method of cleaning the gas cylinder, the brush was for that. It's in the book. CFP 318(2) It was supposed to go in alone though, without anything on it(cloth)...
Our armorer had a word of advise..."If you do it, that's one thing. Bring it to me. Don't do more and damage things." So that's the rule I lived by in later career. Problem is, I'd take the heat for the young soldier...Oh well...