Inquiring minds would love to know about a practical solution. You can add more material, but that's extra bulk and weight. I think we can agree that a mountain is bullet proof, but that's not really practical.

Also, rifles are really good at shooting through shit. We're talking rifles from 1890s. People don't realize that a bullet from a rifle punches through steel plate (and then buy steel body armour like the plebs they are). Modern ar15/ak rifles are wusses by comparison- we traded big badass bullets for smaller ones that are lighter and easier to carry.

More awesome weapons like the rpg-7 basically create a stream of molten metal that goes right through armored vehicles. Or you use depleted uranium or tungsten, which get more dense as they heat up from friction of punching through things.

In the near future we'll have even more badass things like railguns that will go through meters of steel/ceramic/admantium.

So yeah, that mountain may not save you. Meanwhile, mars will be the same.