I don't think a Leica will stop bullets and hammer nails without any ill effects, but they're tough....up to a point. The rangefinder system is a weak area. A hard knock will put it out of whack, as many of the posts above indicate.
But hey, what kills a Leica will kill most other cameras, too. Had one of those vaunted Olympus E-3s, supposed to be a super-tough camera. It fell 6 feet off a tripod onto stone pavement (my fault). Lens broke off clean at the mount, both camera and lens looked to be intact--hardly a scratch on either.
Took 'em in for repair. Three months later, told they were fixed and ready. However, it became obvious after the first few exposures that the focus was way off--despite what the focus confirmation light was saying. Took the camera back in. A month later I was told to come get my camera. I get there, and am handed a BRAND NEW camera and lens as a replacement. Apparently the problem could not be fixed, no matter how they tried, so I got new gear out of it. Very nice of Olympus, I might add.
And this was on a camera billed as being made for rugged, professional use, and which did not appear to have any significant damage (actually worked for a while after being dropped). So tough is one thing, "indestructible" is another....