But would it not be possible to make a pistol with capability to fire plastic ammunition designed specifically for that gun in order to make the cost of ammunition for that gun much less than what it otherwise might have been?
No not really why well because you have to pay a whole bunch of smart guys to figure out a way to fix the problems that it would have then you have to either add another tooling line or retool a line you have. So in the end it was cost much more than just making brass, alum, steel cased ammo...
It is not so much the heat as is the internal ballistics and the other smart guy stuff. Smart guys make big big buck and to have a bunch of them working on a project that will more than likely fail to get off the ground is not a good business practice. They want to make money not lose it.
OK Remington tried that with their etronx rifle and its electronic primers yeah didn't work because the rifle cost $1000+ and the ammo was well over $100 the last time I seen it being sold. $100 for 20 rounds of 22-250 even rich guys are not that damn stupid. Now if you have that rifle you can't shoot it at all because, the last time I looked no one makes ammo for it. But everyone makes 22-250, 308 and 30-06.
Just like the 22 jet and Smith & Wesson. It failed and failed hard.
It is better to make a pistol or rifle to fit existing ammo than to reinvent the wheel to fit the wagon.
I understand your thinking because plastic is cheaper than brass but the bottom line has much more than the price of the raw materials involved.