Here, we see an "exploded" view of a W.B. Niemi Engineering core-seating die. At the top, is the die body, which consists of a "carbide" insert, which was press/shrunk fit into the tool steel, threaded body. These dies, like the original B&A swage-up dies, are threaded 7/8Thsx14 TPI, thus screw into any standard reloading press.
Below, from the left, are the working parts and materials: the reciprocating stop/ejection shaft; the guide/positive STOP "nut"; a spacer, which is used to "adjust" the stop depth (lengthen/shorten the stroke); the FIXED "collar" which is the primary shaft "stop", limiting the reciprocal range; a J4, thirty caliber, 0.925" long jacket; a formed core - in this case, for a 112 Gr. bullet; a W.B. Niemi core-seat punch - probably about .2695" in diameter (+/- 0.0005" would be the "normal" punch range for this combination of jacket and core VOLUME).
The correctly cored-jacket will have COMPLETELY filled the die cavity: this requires substantial pressure. At peak pressure, the fit between core length X diameter, jacket wall thickness, and punch diameter should accommodate a small, but uniform amount of lead to "bleed" around the punch, assuring that no air is trapped between the jacket and core, while the cored jacket is expanded-up to the full diameter of the die cavity. (Note: COMPLETELY full is not actually possible - a radius will always remain at the bullet heel.) Lacking a selection of punch diameters, one must then make a core length X diameter (thus weight) which "matches" the punch diameter and jacket wall thickness (this varies from jacket "Lot" to "Lot"). Within reason, actual bullet weight is the least critical attribute - except for recoil, there isn't any difference between 100 and 116 Gr. bullets . . . or, anything inbetween. RG