Do NOT use .30-40 brass to make .6mm Lee Navy.

[Bruce_in_Oz]

Any time you size necks DOWN to form a new case, eg. ..308W to .260Rem, or worse, .243Win, you need to keep a close eye on neck thickness. As the outside diameter is squeezed, the brass flows to make the neck relatively thicker. If you use the expander ball, you will end up with cases that will be a very tight fit in the chamber neck, if you skip the expander, the "hole in the middle" will be too small for proper seating of the projectile and will bulge alarmingly and not necessarily concentrically when you force in a projectile..

You MUST either ream the inside or turn down the outside of the neck to specified thickness. This also tidies up neck concentricity. I prefer inside-reaming as it removes any "interesting "do-nuts" or internal bulges resulting from movement of brass at the shoulder/neck junction.

And then there is neck-annealing before or after these modifications……

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[browningautorifle]

You MUST either ream the inside or turn down the outside of the neck to specified thickness.

You also wind up with thicker brass up top of the case. Reduced capacity...anyhoo...it was a small bomb.