I'd like to paint one of my rifles, but sitting at 4K+ with NF glass, I'm a little hesitant. Spray paint is removable, but I've read it can still stain the metal even after removal.

Does anyone have any pictures of what the stained metal looks like after Rattlecan removal?

I don't have pictures but it usually is a whitish ghosting residue left behind that if you work at it generally still can be removed.

Yup. BUT an extra wipe with a rag soaked in acetone followed by a good oiling will fix that.  The removers strip all oil off the surfaces, so they get that kind of 'chalky' look.  

Big thing to remember is that it is kind of a pain to get paint out of every nook and cranny after painting.  So while it isn't permanent, it can be a challenge to get every last bit off, in case you are OCD.