True Lies automatic weapon on stairs: Not surprising this didn't work. Nothing about that horrible movie made any sense anyway. And it stands to reason that weapons would be designed not to go off if bumped.
They should have tried that with an early model British Army issue SA-80 from the late 80s/early 90s - the L85A1 - which absolutely would go off if dropped on its butt-plate, even with the safety on! (Prompting several expensive recalls and redesigns)
It couldn't have kept firing as went, like in True Lies, though - for extra shits and giggles it was very prone to jamming when set on auto, and the magazine release stuck out to catch on clothes, so that the clip would drop out at inconvenient moments. The whole thing was a Johnny English scene waiting to happen...