I also think that Fireballs, Contenders, ect are sawed off rifles. As Shokwave type guns are sawed off shotguns.
They are, but they "aren't". The issue goes back the rather screwy definitions in the NFA 34 and the hugely flawed Supreme Court ruling in
Miller vs, US that upheld it.
the guns you mention do not meet the legal definition of rifle or shotgun. They do meet the legal definition of handgun. And nowhere in the law is the term "sawed off" to be found.
Personally, I don't consider them to be "sawed off" anything. They were never made longer, nothing was sawed off.
I realize this is a distinction without a difference to most people and casual conversations, but in law, it does make a difference.
Take an existing rifle or shotgun, and shorten it below the Federal legal length (barrel or overall length) and you have created a "short" firearm that falls under the NFA regulations and requires a Federal license, special investigations and tax in order to possess. Failure to do that is a FELONY.
Create "from scratch" a short firearm, one that NEVER had a stock, or a rifle/shotgun length barrel, and you have made a handgun, even if it uses a rifle or shotgun action, its legally the same as all other handguns, and not an NFA regulated item.
why don't we see new handgun cartridges popping up more often? I can't really think of many (if any) new cartridges since the 90s that have attained widespread use.
The main factors at work here are both mechanical and consumer driven. There are a wide variety of rifle rounds, big, small, short, fat, etc. that fit and work fine in a lot of existing rifle actions. There is less versatility in common handgun designs.
Also a large segment of the handgun market is driven by police and military use. And that influence extends deep into the personal protection market as well. The sporting market is a much smaller segment of handgun buyers than it is of rifle buyers.
One thing you failed to mention is how many of the "new" rifle rounds become widespread common use rounds? Not many. New cartridges often sell well, at first as many people will buy one to check it out, but what are they doing half a dozen years down the road?? Usually not much, sales wise.
The firearms industry has its own versions of fads, fashion, and style. New and different only gets you so much, and once the honeymoon is over it becomes a matter of practicality, PRICE, availabilty, and performance, and new cartridges that don't provide some clear advantage in ALL these areas usually fade rather rapidly.