I have a rifle with a wildcat chambering. The round is called - stamped on the barrel - Scott's .30 Imp(roved). It is a variation of an 'improved' .30-06 Springfield round. The neck is shortened by moving the shoulder forward a bit and changing the shape of the shoulder by making it more of a 'step' than a shoulder.
A regular .30-06 round will headspace in the chamber, so fire-forming is pretty easy. I found some dies for a .30 Gibbs which neck size the case and re-size the main body just enough to chamber.
The advantage is in reloading. It has about 7-10% more capacity than a 'normal' .30-06, so it holds more powder; this allows a larger charge of a slower powder and more velocity. It is especially advantageous with heavier bullets. This rifle loooooves 200 and 220 grain bullets.
And I must confess, it's fun to have an out of the ordinary rifle as well. The rifle itself is built on a 1917 Eddystone action and barrel. The stock has been replaced with an aftermarket by Fajen or some such vendor. The trigger is also an aftermarket and very nice. I put a Leupold scope and mounts on it. It is a very effective .30 caliber rifle - somewhere more than .30-06 and a bit less than .300 Winchester Magnum. I can get a chronographed 2800 f/s with 200 grain bullets without damage to cases.