Originally Posted by
Bazoo
For plain based cast bullets, you want the expander to be .002 smaller than the bullets you intend to use. Unless very hard bullets are used, the brass will swage down the base of the bullet with any smaller of an expander. This can cause inaccuracy and leading, especially if the entire base driving band is reduced in diameter.
For jacketed bullets, a smaller expander is okay since they cannot be swaged down by the brass.
Likewise, if you use a gas checked bullet, you can get away with a smaller expander because the gas check resists being swaged by the force of the brass case pressing against it from all sides. I use a lee 432-265-RF that is longer than any expander I have, that falls into this category. It shoots darn good though.
An M die or stepped expander has a short step just before the flared portion and creates a slight step inside the case about 1/10" tall. This step is generally .002 larger than the intended bullet diameter to be used.
Stepped expanders make seating bullets easier as they create a sort of funnel at the top of the case. When placed in the case, the bullet sets on a small ledge, keeping it aligned perfectly for the seating stem. They work well for not only cast bullets but for plain based jacketed bullets as well.