Hi,
I use a 1/2" gouge, a 1/2" shallow sweep, and a 12" long gunsmith's rasp by Liogier. Here is my apprentice, Maria, using a shallow sweep and her gunsmith's rasp on a fowler.
This Gunline round barrel bedding tool is also very useful for straightening and smoothing the radius.
Round scrapers also help. Let me share a detail. Don't make the radius or crease such that the handle of the baluster wrist (which is what this form of butt stock architecture is called) looks even in height throughout its length. Instead, angle the line of the radius down slightly toward the toe of the stock so the handle gets a little thinner toward the rear. The photo of Maria using the rasp above shows that downward angle.
dave