what is a wildcat cartridge???
#1). There are "factory" cartridges that are made by a factory and ammunition is available over the counter. Much of the development in this area has --in the beginning, .50/70 gov. eg.-- centered around military cartridges and guns sold surplus. 8 x57 mauser. .30/'06 Gov. etc. The factory can be in multiple locations around the world. I forget how many countries have made (with license) "Mausers." Other places without license or a copy (like the 1903 Springfield. Mauser sued and Uncle Sam paid royalties until WW I. At the other extreme, some experimenter comes up with something and finally has a small factory (1 man shop???) and one or two gun stores stocking ammo... it varies.
#2). Improved cartridges. In this situation a cartridge, a "factory cartridge," has been changed to "improve" things. The quickest glimpse at the "turn of the century" cartridges (1900) reveals ALOT of taper. I have had it suggested to me that this was for "feeding" thru the action. The realitively fast powders of the era may also have contributed. .300 H&H is a "funnel shape." Many gunsmiths saw an advantage to expanding the case in the body area to hold more powder among other things. Expanding the body of the H&H (.30) led to many cartridges. Mr. Ackley's is well remembered. The same round with "radiused angles" at base of neck and neck/body juncture is the .300 Weatherby Magnum which was then necked to .25 thru .33... Work, promotion, by Mr. Roy Weatherby. In the beginning an "improved" since you could fire .300 H&H cases in the chamber to make cases for the Weatherby.
#3). Those "on the cutting edge" are working with something that cannot be bought over the counter or easily made like the "improved" case... This combination of changes, usually to a factory brass case, gives you a true "wildcat." At one time the PPC cartridges were quite "wild" 'cause of the difficult availability of brass in .220 Russian from which they were made... Going the other direction, .22 savage Hi Power is on the edge... Norma makes the ammo, but can you get it? What was once a factory cartridge (ask great, great grandpa--someone's) is now on the edge of "wildcat." Newtons. Then there are "dashers" which I understand to be "short chambered" versions of the .243 Winchester in a selection of calibers--I.E. hold less powder... Now you take a short .308 case (parent case of .243..) and you are back to the .300 savage case which is also the .250 Savage and in .24 the 6mm International...
If you really want to wander in this field, Mr. Ackley summarized thousands of cartridges, factory, improved, and wildcat, in two volumes... available from eabco.com, sinclairintl.com, ebay sometimes, etc.-- and the trip is long... luck