I'll simply say I believe there is a reason that a lot of AR-15 manufacturers are pursuing gas piston technology.

If the heat never really gets a chance to flow through the brass casing before it's ejected..or if the casing acts as an insulator, things are good. The heat problem with the existing AR platform is due much more to the gas impingement system heating the bolt and carrier than the cartridge being fired.
This will not mean the barrel itself will not soak heat up, but the chamber is a lot cooler on a piston gun.
Some plastics may be suitable. Whether or not they could be made to fit the existing 5.56mm chamber/throat geometry is a different matter. My gut says probably need much thicker sidewalls, and thicker cartridge web, so it wouldn't work without throating the chamber. Which means likely rebarrelling, as the barrels are chrome lined per mil-spec.
Bottom line, is how much weight is being saved. Is bulk being increased ? End result will probably be a new AR looking weapon
with a different chamber and magazines.

A plastic casing means no reloading. Some of the casing will ablate and change the structure, probably to an unsafe condition.

As an aside, muzzle loaders..look up Harry Melville Pope.

The US Civil War. little known fact. outnumbered, outgunned, outsupplied southerners killed or wounded
30% more union troops than casualties they took during the War of Northern Aggression.
A lot of people might be glad that the south didn't have a lot of Henry, Spencer, and Whitworth rifles. What they did have is people who knew how to shoot and scoot.

For what it's worth, I have a degree in chemical engineering, and I work with certain companies in the firearms industry.

I know I'm so far off topic that I'm going to suggest starting a new thread for future discussions about this.