using a v-gouge maybe? I was hoping to use a scratch stock. Was thinking maybe laying a long metal strip along the bottom as a guide, with a quarter inch shim at the but plate. Not sure that'll work...for that matter not sure it won't.
A scratch stock is kind of iffy performance wise, especially if you are not used to using one, and, it will only do a parallel line which you do not want.
I mark the beginning up near the triggers then the rear and use a #2 pencil to draw the intended molding line. Don't like it? Use the other end of the pencil to remove it, and redraw your line until you do like it. Hold your pencil at an angle to the wood as you draw, you only want a faint line, try and not let the pencil point leave an impression in the wood.
I then very carefully use a sharp knife along my line, lightly, it does not need to be a deep cut, you are only scoring the line. Next is a vernier, a V tool used with a 1lb wooden mallet driven lightly and slowly along "up" side of the line. After that I use a checkering cutter to even out the cut and deepen it. You should now have the beginning of a nice even butt molding tapered the way you intended.
Another way to get that initial line is to clamp a thin strip of flexible wood (any material that works, plastic, whatever) in the butt plate area, then just flexing the wood with one hand draw your line with the other.
dave