It sounds as if you have a small rounded area to fill. This is commonly caused by improper sanding at a corner of the wood. The rounded area you need to fill is not a good shape to support the filling material. The void need to be cut so that the sides are square with the receiver. You need to make a rectangular void about 1/16 X 3/16 with the bottom and side square with the receiver and the back parallel with the receiver. This can be done quickly with a Dremel Tool or a good sharp knife. Miniature chisels are also good and can quickly be made with a grinder. Make all cuts toward the receiver end to prevent splitting the wood along the grain.
Next, with a sharpened ice pick or similar tool, with light hand pressure make several indentations in the wood. A bent safety pin can also be used for this. This will give the filler something to grab on to as it hardens.
I prefer epoxy, but a good Bondo product will also work. The epoxy will flow into the small irregularities of the wood much better than bondo. Coat he receiver with oil to prevent adhesion of the filler with the metal. Fill the void, put on the receiver and let things cure. The excess filler can carefully shaped with a file after 24 hours. With such a small area to fill, an alternative is to put a piece of tape over the receiver end of the wood to make a dam. Fill the void and shape up things after a day. Make sure the filler is flush with the rest of the wood where it meets the receiver of the stock will split as the gun is shot.
If you have the tools and material, the job should be about 20 minutes of fun.
Pat Ireland