Acetone is your best bet with steel wool, sandpaper is going to eat away a lot of the metal itself, and a low grit paper is what you want to use anyways to make bold wear marks.
Just hold the weapon, imagine where your hands would be during different stances, and wear away those spots. Also spots that stick out, or are likely to rub off against something are worth sanding off, it is a pet peeve of mine to see a gun with wear marks in a spot where it shouldn't be, so just be smart about it.
Just get some nail polish remover from your mom or some rubbing alcohol out of the medicine cabinet. Either will be better than sandpaper as it'll tear up your receiver. As mentioned, make sure to wear it in the high traffic areas only as nothing is cornier than the guys who pre-wear their guns in the wrong places.
Ok I do have nail polish remover available, not for me hehe but my moms. So I am going to go for a tan and lighter brown for an A-TACS look and not be corny and wear it with the polish in the right places. www.youtube.com/watch?v=TqHf1uWbXtQ That's the video I am following.
Along with what everyone else is saying, I've used brake clean. You can get it at any auto parts store and it works like a charm! It pulls the paint off with less elbow grease than nail polish remover. Also I'd say try random "not super abrasive" items to remove the paint. I've used several items from a piece of motorized floor scrubber brush, to an automotive moldable sanding block.