This video humorously discusses the evolution of the term "mess" in the military, showing how it now refers to social and professional centers within units, not just dining halls.
Article (and the mag it came from) is in German in this PDF from HK's site and hasn't yet been translated by them to English, but here's an article about the HK421--and this version (unlike others seen so far) does have a safe/semi/full auto selector switch: