bunch of arseholes... most of them
The truth is that there's an exceptionally high magnetic resonance field that messes with artificial joints and drops light loads such as handgun bullets- why do you think they don't allow rifle ammo on the indoor range?
No, seriously, they're thick plates of steel that are angled to absorb impact, without penetration and generally arranged to keep stray shots from exiting the building. Not always effective, our indoor range has hollow metal doors (16 gauge frames and 18 gauge doors) with holes and dents all through them.