The reforming process starts with US made .303 Britishbrass. This brass is about .450 to .452 in diameter just ahead of the rim.
You might find a handful of once fired or multi-fired .303 cases to use for process development. Once you have the process mastered it is pretty easy to get 100% yield when forming new brass.
These are Hornady Frontier .303 cases. The 2 outboard cases in the lid are new and untouched. The next pair moving inward are cases that have been formed with 4 dies. Most people can form with 3 and some brass will form with only 2 dies. Forming failures when developing the process usually result in the case folding like an accordion at the shoulder because the die cannot support the small body diameter of the .303 case.
Pick on the photo to enlarge it.
Attachment 72588