[@Ollumhammersong]

True, the mechanicus doesn't approve, but he's an average guardsman. To him, the whole 'you should just let the enemy shoot your vulnerable ass face because we don't put any armour there," is bullshit, and he would rather have a protected noggin. I imagine the armour isn't nearly that of an average armoured sentinel, but it is better than a scout sentinel. Maths has been sorted.