"Our Astartes are true heroes all! Noble, brave, loyal, and above all completely rational and not at all given to fanaticism and superstitious thinking."
"I agree my liege."
"And where do heroes get shot?"
"...sorry?"
"In the shoulder, lad! Heroes always get shot in the shoulder. It's the damndest thing, but because I am a higher being and the most perfect human in all of creation that ever was and ever will be, believe me when I say it is true: Heroes will be shot in the shoulder a thousand times before they are ever wounded in the torso."
"...I suppose I have to admit that I have witnessed more desperate, brave men clutching their shoulders as they save the day at the last possible second than I have seen such men holding their organs into their stomach while doing likewise."
"And it's always men."
"Oh goodness, yes. Perish the thought."
"So take this back to the Mechanicum and don't come back until you have a design that will protect the heroic shoulders of my proud Astartes from any conceivable threat."
"And what about their heads? This helmet design?"
"Oh, heroes don't wear helmets. Just do whatever."
"At once my liege!"