The X-Punch is one of two "secret ingredients" in the manufacture of high performance, low velocity (such as sub-sonic) projectiles. It can also be used to make low-penetration, high-fragmentation (frangible) handgun and rifle bullets, shotgun slugs and airgun pellets.
(The other ingredient is the SDD-1 Serrator Die.)
The cruciform tip or X-shape is machined into the end of an external punch (top punch in Corbin presses). The punch fits the core swage die, and separates the core into four quarters, attached by the solid base section. The usual design divides from 1/4 to 1/2 of the total core length, although punches with longer X-sections can be made to divide the core nearly all the way into four quadrants.
Lubricant is wiped or sprayed into the X-shaped "hole" in the core, and then the core is seated in a jacket using either a normal open tip punch or a conventional hollow point punch, which presses the lead sections back together but, because of the lubricant, they do not join as a solid again. This "pre-fragmenting" of the lead core allows the core to open more quickly on impact into four equal sections, especially when coupled with jacket serration (thin cuts in the OD of the jacket made by pushing the jacket through the SDD-1 die, which has a "crown of thorns" arrangement of cutter points to score the jacket, and a reducing section to iron the slight ridges back into the groove, leaving the jacket weakened along six stress lines so it peels back evenly on impact).
The X-punch may also be used in the core seating operation, instead of core swaging. A somewhat lighter than usual core, in relation to the jacket length, is seated into a jacket using the X-shaped punch, which extrudes four quarters of lead between the blades of the punch. The a small amount of lubricant is sprayed or wiped into the opening, and then the ogive is formed on the bullet. There are many variations on this design, including seating the core with a hollow point punch and then changing to the X-punch, or following the X-punch with a hollow point punch. Each method produces a slightly different effect, both in appearance and in performance. But the general trend is to pre-fragment the forward part of the core, from a quarter to half of its length, so that the bullet tends to open up faster and more evenly.
The advantage of the CORE SWAGE (CSW-1) method is that a single core swage X-punch will work for various calibers of bullets that can use the same core diameter. For example, the .308, .311 (303 British), .312, .314, and even the .318 and .323 (8mm) may all use the same .251-inch diameter swaged core depending on jacket wall thickness. Therefore, a single core swage die and X-punch would work for all of them. But the advantage of the CORE SEATING (CS-1) method is that the X-shaped cavity can be left more visible, since the spacing between quadrants of lead does not have to be pushed together unless you want to (one does not have to follow the X-punch with a hollow point or normal core seat punch). The Core Seating stage, of course, has one established diameter or caliber, so it is not as versatile as the Core Swage in regard to use with other sizes of bullets.
The X-punch is a "custom" punch in regard to pricing because of the amount of extra time required to machine, polish, and fit the blades of the punch compared to a conventional cylindrical punch. The depth of the blades may be specified, or left to the discretion of the die-makers, as you wish. More bullet makers tend to get the core seating size punch (the only real difference between the two types is the diameter: the core swage punch fits the bore of the core swage die, and the core seating punch usually fits the inside of a particular jacket). It makes a more visually appealing design, which helps to identify and market the bullet, as well as giving it more open area to aid in rapid expansion.