A set of dies are generally one of three different types– full length, small base or neck sizing dies. Neck sizing dies size only the neck of a case. These are used for cartridges that are to be fired in the same rifle time after time without having to work the case any more than necessary. Full length dies size the case back to smaller than fired dimensions, but not all the way back down to its unfired dimensions. This is the most common die found, and usually used on most bolt action rifle ammo. With their terrific camming power, a cartridge can be easily forced into the chamber even if it is a bit snug. Semi-autos and pumps do not have this type of camming power, and as a result, require the case to be resized somewhat smaller than what the full length die can do. This is exactly the type of situation that calls for the small base sizing die.