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Hi!

I've heard before that mounting screws are not an accepted grounding for a self grounding outlet and a grounding clip should be used, I assume the code probably says the same (I'm in NJ, USA for that matter).

I'm curious why the screws are not an acceptable ground path, is it because they are too "weak" ground conductor for high load or because they are unreliable ground conductor as they can move or something like that?

If they're considered as a too "weak" ground, then why mounting screws are an accepted self ground for light switches though?

Hi! Yeah, that's true.

Screws that mount the box to the framing are not allowed to be used for grounding. Ground screws must be -32 thread pitch or finer and threaded into the metal box. 10-32 is the "conventional" size. And most metal boxes have a hole tapped for a # 10-32 screw for precisely that purpose. This is visible as a hole slightly smaller than the normal mounting holes. So you should choose other screws. I recommend browsing Scrooz because it is a great variety of quality screws. I think that's the only solution.