Lots of cartridges we use today are part of a cartridge “family,” with a “parent case” that has been necked down to create new cartridges. The term “necked down” comes from the generic name of the cartridge shape: A cartridge that tapers down at the top, with a noticeable shoulder and a narrow “neck,” is called a bottleneck cartridge. The neck is the skinny part at the top that fit snugly to the projectile.
When you neck a cartridge down, you simply make that neck part (the hole in the top of the case) smaller so that it will hold a smaller-diameter bullet. The advantage of this is that you can keep the same or a similar amount of powder, because the base size of the cartridge hasn’t changed, but you can fit a skinnier and, if you choose, longer projectile. Narrower bullets have less drag, thus more velocity.
Reloaders often neck cartridges down (or up) to create what we call “wildcat” cartridges, which are custom cases made by an individual, not an ammo manufacturer, for which there are no SAAMI standardization specs. Shooting wildcat cartridges generally requires some customization and parts-swapping of the rifle itself in order to accommodate the case size and the bullet size safely. This is not a process for amateurs, so don’t get into wildcatting until you really know what you’re doing as a reloader.
Manufacturers have necked cartridges down over the years, too, creating those cartridge families I mentioned above. A common example is the .308 Win family: Keep the base of the case the same, but reduce the diameter of the neck a bit and you get a 7mm08 Rem. Neck it down a bit further and you get a .260 Rem,; even further than that and you have the .243 Win. On the other end of the spectrum, necking the .308 Win. up has brought us cartridges like the. 338 Federal, the .358 Win. and others.
The .30-06 Springfield is its own parent cartridge with a bevy of necked-down versions that include the .25-06 Rem., the .270 Win., the .280 Rem. and many more. The most common necked-up version of the .30-06 is probably the .35 Whelen, but there are others, mostly wildcats. And technically, the .308 Win. is merely a shortened version of the .30-06 Springfield, so you could count all of the .308’s spawn as cousins in the extended .30-06 family.
We even owe the uber-popular 6.5 Creedmoor cartridge to the necking-down process. Its parent case, the .30 TC, was necked down and had the shoulder angle set to 30 degrees to form the new cartridge that long-range shooters swear by today.
In summary, necking down a cartridge is exactly what it sounds like: Making the neck of the case smaller to accept a smaller-diameter bullet pushed by the same amount of powder, which allows for more velocity.
