Sinclair International has a helpful, three-part video series on Full-Length Sizing. The full-length sizing die performs multiple important functions: it resizes the case body, resizes the neck, and adjusts the headspace relative to the chamber (it can also eject the spent primer if that was not done previously).

While neck-sizing-only can work with moderate loads (for a couple firings), after repeated firings the case can stretch, becoming too tight to chamber easily. If you shoot cases with high-pressure, near-max loads, you will probably benefit from full-length sizing your cases each reloading cycle. When full-length sizing, you will want to move the shoulder back (i.e. “bump” the shoulder) to provide proper clearance in the chamber. A case that has grown too much will exhibit stiff bolt lift after firing and be hard to chamber if it is not FL-sized during the reloading process.

Sinclair recommends bumping cases .001-.002″ (one to two thousandths) for cases used in bolt-action target-shooting rifles, or .003-.005″ (three to five thousandths) for hunting rifles or semi-auto rifles. To move the shoulder back you screw the FL-sizing die downwards in the press once you’ve determined “just touching” on the shoulder. You don’t have to screw the die down very far! With a normal 14-pitch die, 1/8th turn (45° rotation) yields approximately 0.009″ of downward movement. So it doesn’t take much to add a few thousandths of bump.

Case Sizing Part One — Why We Full-Length Resize

Case Sizing Part Two — How to Set Up Your Sizing Die

Case Sizing Part Three — How to Use a Bump Gauge

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