One popular commercial filler is Behlen's Pore-O-Pack. It is an excellent filler for open grain wood, but is not the best for gunstocks because the main ingredient is silica (sand) powder.
This makes it very hard on checkering tools unless you use carbide.

I use the sanded-in method as several above have mentioned and it works to perfection if you have the patience.

http://www.woodworking.org/WC/GArchive99/1_20waltcunfin.html

RWO