According to Sarna in their
BAR section's material analogue list, BattleMech Armor is equivalent to solid rock.
(Having worked in archaeology with a lithics specialist, I'm familiar with the Mohs Scale of Hardness, so I know that not all solid rock is created equal. However!) Curious, I went digging on YouTube to see what kind of damage bullets could do to different kinds of rock.
I found one from Kentucky Ballistics where he shoots at an
Atlas Stone: a giant ball of concrete made for super-heavy weight lifting. It's concrete which isn't as hard as granite.
But, I think the one that answered the most questions for me was this one by Edwin Sarkissian, where he
fires at an honest boulder in the wild with a range of rounds.
I know what I'm taking away from this. Have a look for yourself.