From a naysayer;
.303 Savage .30/30 Winchester
Neck dia. .333 in. .330 in.
Shoulder dia. .413 in. .401 in.
Base dia. .442 in. .422 in.
Rim dia. .505 in. .506 in.
Rim thickness .063 in. .063 in.
Case length 2.015 in. 2.039 in.
As you can see the .30/30 case is generally smaller but is greater in length. All you are doing is fireforming the .30/30 case to the .303 dimensions.
Also the longer .30/30 case is not good in a shorter chamber. There is a reason you are not supposed to fire ammunition in a rifle not stamped with the same calibre designation. Still remain a naysayer just the same.