At least shooting rocks was fun!

[Daemion]

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. 

[Prospernia]

Looks like he's in the Mojave-desert firing at what looks like a igneous-rock, probably granite.

If mech-armor is like that rock the maybe the weapons in Battletech, 3025, are superb; however, I doubt that given the guns in 3026.

[Daryk]

The anti-armor rounds he fired drilled that rock though... ;)

[ANS Kamas P81]

Be careful shooting at rocks, ricochets can be dangerous.

[MoneyLovinOgre4Hire]

That person's lucky that ricochet wasn't a few inches to the side.

[PsihoKekec]

And unlucky that ricochet went all the way back to the point of origin, in 99,99% of cases ricochet goes to the  side or upwards, rear ricochets are very rare (shattered bullet notwithstanding, pieces of those go everywhere, but lose energy fast).