What is this??

[shooter71]

the opening on the stock, what is that ??

http://www.budsgunshop.com/catalog/product_info.php/cPath/37_336/products_id/46285

 

[Rimrod]

It looks like a "speed feed" stock. Which carries extra ammo inside the stock and feeds them under spring tension, like removing them from a magazine tube.

 

[sceva]

The hole is the speedfeed feature. It is the opening to a spare shell holder.

sceva