Brake cleaner removes paint well too.

Are you wanting simple and inexpensive like a rattle can job, or something more involved and costly?

Remove the rust. Either remove the scope (if it has one) or cover the lenses. If Irons, mask them or remove them. Wipe it all down with acetone. Krylon brand camo dries very fast, which is a help since you layer paint colors. O.D. green is as dark as you want to go if you want camo. Black is terrible for camoflage. It's easier to add dark color to light than light to dark. Spray the whole thing tan, then you have options. Cut tree branches and grass for pattern, or (one of the more effective camo methods) get a coarse sponge, and slowly add a random sponge pattern to the tan in greens and browns.

If you don't like what you see, copious amounts of acetone removes everything you did and you can start over.

Once you're satisfied with it, give it a day or two, and coat it all in flat clear coat. It'll hold for several years. And you've only got about $15 in the whole thing.

rattle can job