If I understand correctly, and the new wire is to run out of the existing metal box, to another outlet "downstream", you would just connect the new green ground wire to the box since the box is grounded and the existing outlet picks up the ground via its attachment to the box.

I believe you understood it correctly. So for wiring then with MC its:

1. New outlet hot to existing hot
2. New outlet neutral to existing neutral
3. New outlet ground to existing outlet box

That would be to code and no issues? If I have it popped open already and the wire on hand should I bother wiring the existing outlet to ground or is it just irrelevant?

For the wiring, yes. That is correct.

If you are showing a ground on the existing outlet with your tester, it just gets its ground from the box. It isn't necessary, but it would be more clear to the next person if you connected a short pigtail ground wire from the box to the current outlet's ground screw.

If it is your first time working with armoured cable, don't forget to buy the matching red plastic bushings to protect the wire along with cable clamps/box connectors that can accept MC/AC90 wire

Thanks - I went the lazy DIY mans route. That item I bought I linked in the original post is an all in one deal. It's 12' and has a snap in fitting with the red bushing already attached at the end. All I should have to do is connect it to the boxes and hang it.

Ah ok! Didn't even know they sell that

The outlet has a ground connector in your first picture, the bottom right.

That should be connected to the new ground (green) wire in the flex whip. You can wire nut all 3 together and zip a screw into the conduit box for even more protection, being the conduit as a pathway to ground to your panel.

True, flex conduit should not be used as a path for ground, hence why there's a ground wire included in the whip.

Thanks, That helps. I think this should be a lot more straightforward than I originally thought.

I knew I could wire the new outlet to ground in the existing box through the green wire and have a solid ground. I just didn’t know if there were any reasons not to. I couldn’t think of any.

Also since that current outlet already has a ground without a ground wire, I was wondering if I should wire it up while I had it open. I’m going to.

I have the ground screws, some green wiring and the green ground twist connectors.

Thanks for confirming I was on the right path. It’s been awhile since I’ve touched electric, and I know that just because it works and tests right doesn’t always mean it is right.

Hey OP, there's a great little unit that can test proper wiring and also test GFCI outlets. To me, they are a must for anyone DYI'ing home wiring, or if a previous home-owner has done some of their own wiring work. Best part is it's under $13: https://smile.amazon.com/Receptacle-Tester-Klein-Tools-RT210/dp/B01AKX8L0M?th=1

Thanks. I have one similar to that, and it's showing good ground on what the previous owner did.

I knew I could get good ground through the green wire on the MC cable, I just didn't know if there were any issues with changing the "medium" (for lack of a better word) of ground on a circuit or if there was a better way.

The current outlet was grounded through the conduit, but the whip I bought was MC and that sheath shouldn't be used for ground. So it will be ground through the conduit up to the point that the homeowner stopped. I'll attach the green wire to that existing box to ground my new outlet.

So when I'm done the ground will look like:

(New Outlet) --green wire--> (Existing Outlet Box) --conduit--> ground

I was trying to figure out if it was ok to do half a circuit grounded through one type and the other half another.