I recently painted a couple of stocks after I modified them for my boys to use. I sanded lightly, then a couple of coats of Rustoleum primer, the Rustoleum textured paint. Tunred out great.
Did you have to clear-coat the Rustoleum? Also, Rustoleum (as most any other oil-based paints) adds quite a bit of volume to the project (paint expansion); did you find this to be a problem when fitting the parts back together?