Brass is wonderful, malleable stuff, and about 30 seconds after the first brass cartridge saw the light of day, someone undoubtedly beheld that case and thought that if they could modify it to get a little more powder in it, they could really get the sumbitch cooking. Thus was wildcatting born, and The Great Wildcatting Boom lasted roughly from the end of World War I until the mid-1960s. During that time, factory loads were necked up, necked down, given different shoulder shapes, less body taper, and endowed with fanciful names. Many wondrous claims were made for them, and the shooting public lapped it up.
The most popular approach was to “improve” a cartridge. It involved firing a factory case in an improved chamber, which caused the case to re-form itself with a sharper shoulder, less body taper, and somewhat greater powder capacity. The wildcatter would then stuff it full of powder, shoot it, listen to the ear-splitting crack it made, and claim whatever velocity he felt was appropriate.
Until the 1970s chronographs were rare indeed, and these claims went unchallenged. But as time went on, chronographs became simple and handy, and they took most of the steam out of the wildcatters. Almost simultaneously, ammo makers snapped out of their lethargy and began producing new cases at a far faster rate than anyone could use them. This is still going on today.
If you want more velocity than you already have, you must look for at least 200 fps more. Anything less is ballistic onanism. And the only way to get that kind of an increase is by burning a lot more powder. To do this, you need a bigger case. So, if you have a .270 and need more speed, you do not torture it into a slightly different shape. You buy a .270 Weatherby or a .270 WSM.
Wildcatting was not altogether futile. Roy Weatherby’s line of cartridges were once wildcats, as was the .22/250, .35 Whelen, the .25/06, the .243 and 6mm, the .257 Roberts, and on, and on. Wild claims or no, they were good, useful cartridges and they’ve lasted. Most of it, though, was just a lot of hot air–but people had fun, and that’s what matters. This is, after all, a hobby.