Here's a load for a 12 gauge that is about as easy on your shoulder as it can be. One ounce of shot @ 980 fps.

I don't know if it will cycle an autoloader, but it works well in pumps, etc...

http://www.basspro.com/Winchester-AA-Low-Recoil/Low-Noise-Target-Loads-Shotshells/product/67999/