Got a question for you guys that reload why is it that a 416 Rigby cant be load to achieve the same velocity as the 416 Weatherby arn't they the same size cases?

You can basically do what you are talking about, and that is how I load for my elk rifle. The only difference is that the web on the .416 WBY cases are usually a tiny bit thicker than a standard .416 Rigby case. Most reloading manuals always load for the lighter standard Rigby load: 400 gr bullet at 2400 fps (for old rifles, in the HOT African environment). If you purchase a Barnes manual, it will show you how to safely "hot-rod" the venerable Rigby.

WELCOME TO THE FAMILY, BTW!