For hydrographics/water transfer printing, paint thinner will take the pattern off very easily if there is no clear coat applied yet. Otherwise it will take a little longer and more paint thinner.
As for other kinds of paints, (I've used Krylon and Rustoleum), paint thinner, goo gone, Winchester brand gun cleaner, brake cleaner have all done the job in varying degrees. It's easier to get off before the paint cures.
If you strip the finish off the scope I would use an engine flat black and then a matte or flat rattle can clear coat by Valspar. At Lowe's, the Valspar brand has an exceptional plastic primer (adhesion promoter) for items like rifle stocks. These are usually ABS plastic but sometimes can be polyethylene or polypropylene which are mixed with a release agent to get them out of the mold.
The release agents work against paint adhesion. Another solution to draw out the release agent is to take a Bernz-o-matic torch on the lowest setting and pass it over the surface of the item until you see a slight shine taking place. As soon as you see that, take a rag and pour some denatured alcohol on it and wipe down the surface.
Any questions, PM me.