Test a bit of whatever paint remover you choose on a non vital inconspicuous place. take the butt plate off and dab a little bit on the inside. Wait a little while and check to see if it caused any change in texture.
Some solvents eat certain plastics like acid, make sure the one you pick does not.
Painting camo on a rifle is done in a variety of ways that have different kinds of results.
Look up some camouflage stencils. print out several copies of each and cut them out with an xacto knife. Paint a base coat of white and allow to dry. Tape stencils down with painter tape. Start transfering the pattern to the areas you want it. Try to get some heavy paper to print out the stencils as you will get less paint bleeding through.