Here is all I know. The rails are removable in case you don't want the extra weight. Two interchangable compartments. The first holds 8 extra rounds while the second can hold a small "survival" kit. (or your cigarettes and lighter)
The stock is carbon fiber. A company by the name of Wild Dog in Australia did it as a proto-type. My understanding that they currently only offer custom bolt action rifle stocks. All their stocks are carbon fiber and all have the fold out cartridge panel. The rimfire model will hold 42 rounds in the stock. The 222, 223 size model will hold 18 rounds. The short action model will hold 8 rounds. They claim that the centerfire ammunition storage hatches feature aspongeEPDM gasket which holds the cartridges firmly in place so that they will not rattle. Theyalso claim that thehatch doors are also perfectly quiet and rattle-free as they firmly locate in either the open or closed position.
Here's a link.
http://www.wilddog.com.au/content/products.asp