As brass is fired and resized it gets "work hardened". The harder brass acts "springy" and will tend to spring back close to its sized dimention. The cases tend to not seal well in the chamber and will usually blow back some soot (dirty case). New brass or cases that have been annealed puff up to near chamber dimentions and are not springy.
Accuracy usually diminishes when brass hardens. The fix is simple, anneal your cases. To do this, stand the deprimed cases in a pan of cold water about an inch deep. Use a propane torch and heat the case from the shoulder to the mouth (one at a time). When hot, tip them over into the water. This will make the brass soft again. Dry and resize, then reload. Annealing will also extend case life and should be done every 3rd firing.
