The device is listed as grounded (and I mean both switches and receptacles) solely by means of the screw contact. It does not have to be in contact with the mudring to comply with the listing. (come on what percentage of devivces actually contact the mudring?...2%....3%? )_
Even when you use 5/8" rings with 1/2" drywall and 3/4" rings with 5/8" drywall the sheetrock installers rarely manage to suck the rock up enough to overcome the protrudeing box screws that hold the rock out a good 1/8"......
Most electricians are scared to even try useing "overdepth" rings for the drywall like I routinely do. Why?...because drywallers are a crapshoot and even when they are doing a great job there will still be a wide variation in how deep your boxes are given modern methods. (zip tools) A mudring that is recessed too far is easier to ignore than one that protudes even by an 1/8th of an inch.....sigh....
Add to that saving money buy not buying bracket boxes and secureing them with all four screw holes.......nope...screwing four-squares directly to the studs with two 45 degreed screws...or the use a spring metal clip in the areas that will still accept such methods is the method of choice....with a usually unavoidable and by code unacceptable increase in mounting depth from the stud face.....
Sigh....stack some washers on the 6/32's or as we have done since way before I got into the trade 25+ years ago wrap some bare 12 around the screws so the receptacle has a solid surface to support it and the plate doesn't split when you force a plug into it.
But anyway I'm ranting off topic....yes the receptacles and switches are listed as grounding....no they don't have to contact the mudring to achieve that rating.
