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Huma
Yes, you are correct in saying you have metal-to-metal ground path in your case. To code? No. As safe as a ground pigtail from the box to the receptacle? No. How much more unsafe? I am not one to judge except that it is not to code.
If you are sure that you do in fact know that your metal boxes are grounded the I would say using a pigtail to a grounding box clip would be your best option here. The cost is minimal; safety improved though.
I am about to write far more than I need to in order to just say "yes, ground the receptacle with a pigtail" only because it is Sunday and I have the time to write a novel while I sip on a beer.
Forgive an old man that has a lot of time on his hand and no one to talk to today.
I say grounding clips because using screws (however less expensive) you may find it much harder to secure the grounding pigtail to the metal box.
If it was just one receptacle you had to deal with either method would be just as easy maybe; but since you are dealing with 20+ I would use a grounding clip (incase).
Several reasons why a grounding clip will be easier or better-
#1 - Attempting to hold the ground screw in your fingers and place it in the back of the box and hold it there while you are able to grab a few threads of the hole with your screw driver will be difficult especially if you have large fingers/hand. I guarantee it will aggravate you. You will not be able to get the screw far enough into the box to meet the screw hole without it dropping into the bottom of the box because you fingers can't get in far enough. Even with magnetic screwdriver it will be difficult. (See below for this method "trick screwdriver" if you choose this method).
#2 - Depending on how the box was hack/mounted etc and what it was mounted to, the hole that is threaded for the ground screw may be compromised/stripped with a sheet rock screw or some other type of screw. So the hole threads could be shot. Also, may not be enough room behind the screw for you to tighten it down far enough to be secured.
What method do I use usually? Simple - in my bag of tricks I have grounding screws and grounding clips. My first choice is a grounding screw. If I find the threading is stripped in the hole I go to a grounding clamp/clip.
"Trick Screwdriver"
I learned very early in my trade to have one of these in my bag of tricks for grounding screws.
It's a bit difficult to see but the tip of the screwdriver is designed to expand and hold the screw steady and outward when the handle is slid forward. If you have never seen one and don't know quite what I am talking about it is almost like "You had to be there" kind of thing. The link I gave you is not the specific one/size you may need but just a representation for visual illustration.
Klein Tools 1/4 in. Slotted Screw-Holding Flat Head Screwdriver with 4 in. Round Shank-K34 - The Home Depot
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