Removing the Old Finish
Sand down your stock to its original wood layer. This means sanding away all previous finish. Applying new finish on top old will result in a glossy, tacky rifle stock. You can hold the rifle in your lap while sanding it with sandpaper. Another technique is to place the rifle in a vice to hold it in place (line the vice with leather so that no abrasions are made on the rifle).
Fine Sanding
You can use sandpaper by hand or an electric hand-sander to remove most of the original finish. This should be done with a rough sandpaper around 100 grit. After most of the finish is removed, gradually work using sandpaper with finer grit (100, 150, 220, 320, 400, 600) to remove small scratches and chips in the stock.
Acraglas
Acraglas is one finish you can use on your stock. Acraglas is a durable laminate that looks like a thin layer of clear plastic is lining the rifle. It is durable and adds a glossy touch to your stock. Applying acraglas is more involved than regular wood varnish and will require you to use a heat source (i.e. torch) during application.
Varnish
Regular wood varnish bought at local hardware stores can be used as well to finish your rifle stock. These varnishes can be made to look dull or glossy; however, you will not be able to achieve the thick laminate look of acraglas with these varnishes. Finishing with varnish is a lot easier and only requires you to rub on for application and needs no heat.