-- excerpt from the article that Bullwinkle mentioned --
The heavy premium bullets in each caliber are long, and long bullets need to be seated in long case necks if the case is to work well in its secondary but crucial role of alignment jig to position the bullet for accurate entry into the barrel when the cartridge is fired. I've coined the term socket engagement for the concept of how much bullet shank the case neck grips. To appreciate the importance of socket engagement as it relates to the exposed length of the bullet, consider the parallels of a fence post, a telephone pole, and a flag pole.
You can get by all right by seating a fence post no deeper than two or three feet in the ground. A telephone pole, because its exposed length is much more than the three to six above-ground feet of a fence post, has to be seated much deeper to be acceptably stable. The much taller flag pole has to be seated even deeper still, or it won't stay vertical very long. Sky-scraper buildings on Manhattan Island are seated storeys deep into bedrock. The socket engagement of the fence post would not be worth much for the telephone pole, the flag pole, or a tall building. Socket engagement that's adequate for the lightest, shortest bullets in each caliber would not be enough for the longest bullets.
Somewhat arbitrarily but with what I considered to be good reasons, I established the neck length at 0.375 inch for all twelve of the new Howell cartridges. If you look at the heaviest premium bullets in all these diameters, you'll see that they're all very nearly the same length. Therefore they all need about the same length of socket engagement.
