I have these really cool cartridege holders that slide right over the butt of my rifles. They hold about six rounds nicely and the ammo is readily available and right under my cheek. I have one that is open and another that has a flap that clicks over the ammo. It looks like both types are readily available on ebay too. Here are the links:

http://cgi.ebay.com/Allen-Rifle-Buttstoc...%3A1%7C294%3A50

http://cgi.ebay.com/Natural-Gear-Rifle-C...%3A1%7C294%3A50

Good luck,

Christopher