For receptacles, the screws are not enough. That is because you plug in loads, and that load could have a rather significant current draw. That is too much for screws. (and this explains why the screws are enough on switches).
However, there is one thing that is enough. Notice how any common receptacle has a metal "yoke". The ends of the yoke may also have removable drywall ears. Notice also the metal box has metal where the screw goes in. If all these things are true:
- The metal box is flush to the wall, and the surrounding wall surface is not proud of it
- The metal parts on both receptacle and box are bare and free of rust, paint or other contaminants
- you have removed the little paper/plastic "squares" that capture the receptacle's screws on the yoke
Then the hard-flush mating of those bare metal surfaces is an adequate grounding path for the receptacle, and you don't need a ground jumper.
Remember most recpetacles have one hole tapped #10-32 for a ground screw. If you need to wire a ground, you can use that hole, use a grounding clip, or drill and tap your own. If you do, it must be -32 thread of finer; random sheet-metal screws are Right Out. 10-32 is the quasi-standard and cute green 10-32 screws are readily available at any building supply, with or without preattached green pigtails. I'm not a fan of the "with" because they are solid wire, which makes it the only solid wire in my work.