Mix raisin bran, flour, sugar, and salt in a VERY large mixing bowl. (This is a very large recipe, be sure the bowl can be covered tightly). Add eggs, shortening and buttermilk. Mix well. Store in covered container in refrigerator. Refrigerate 1 day before using.
To Bake: DO NOT STIR BATTER!! Just fill greased muffin tins 2/3 full. Bake in preheated oven 400 for 15-20 minutes. Just bake as many muffins as you need at a time.