Conical cartridges: stroke of brilliance?

You mean tapered? That's been around since the very early days of cartridges. It makes extraction easier, but obviously loses some potential capacity.

Many "Improved" wildcats are not much more than taking a standard case and blowing out the taper to increase case capacity. The 30-30 A.I. in the pic is simply a 30-30 case fireformed in an Improved chamber which removed the taper and created a new shoulder.



And yes...people have "Improved" the 7.62X39 case...

From a design standpoint, I'm a fan.
7.62x39, 7.62x54r, 5.45x39, and 9mm are the common modern ones I know off hand. I'm sure there are more.

Even a lot of cases of a more cylindrical design have a very, very slight taper to make them easier to form and facilitate extraction.

Taper has been a very important consideration in cartridge design. The most successful military cartridges used in semi automatic mechanisms have a great deal of taper and that is not an accident.

Taper allows for better steering of the cartridge and better extraction.

The Improved cartridges are not really improved, "improved" is just a label created to sell the concept. Like "Greenland". From my web research I found posters who could not get their 30-30 AI’s to feed in their lever actions. Those mechanisms needed the parent cartridge taper to function properly.

Straight sided cases create problems for designers as cartridge feed has to be directly in line with the centerline of the chamber. This is the reason those WSSM rounds are not in controlled round actions, from what I have read, they don't have the taper necessary to function in the magazine or extractor, and I don't doubt it.