Just a thought; I have seen videos of guns run full automatic until the barrels glow (and some melt and sag). In none of these destruction videos does the action or chamber get hot enough to make the steel glow, so most of the heat is directed down the barrel. If some member has an infrared thermometer perhaps they could measure temps of brass, vs steel cases or aluminum cases or even the Shell Shock two piece nickel alloy cases? That would be interesting. I saw the above video quite a while ago, but I have never seen one of the "plastic" cases or know anyone that has, has the Armed Services dropped the experiment?...
Colt did one of those, 6 mags in, the gas tube on a M4 style rifle was red hot, another mag caught the handguard on fire, and then the barrel started drooping and a bullet exited it near the gas port.
The larger mass of the chamber area might be slowing it down, but it is opening up and gas does eject past the brass into the action on a self loader. Hence, dirty brass, whether a blow back, impingement or gas pistol. Also, handguards with ventilation, unlike manual actions.
Motorcycle tuners are known to use a grease pencil and mark down the length of exhaust pipe to "tune" the length, where it stops melting is their indication the gasses have cooled - which also affects velocity of that gas. Thats where they cut the pipe. One a firearm where the barrel is hottest would indicate the maximum burn and elevated temp of the gas residue. It would diminish after that with expansion and cooling. Look to the Thompson's location of cooling rings on the barrel, similar.
The composition of the case doesn't likely change things much at all. And not to forget, artillery and ships guns often don't use a case at all, projectile and powder bags are their thing after a certain size. Huge pieces of hot brass in a tank turret bouncing around would be an issue.
5.56 down your neck is too. Im all for a composition case - the other benefits are real, no brass scrap, policing, sale, etc altho the reloaders will get pinched in that. No roly poly under foot in urban combat if caseless, noise, and the poly casings being less reflective and colorful, too. Reduced weight is a factor, altho anyone who's studied the history of equipping troops knows that a 40% reduction in ammo weight really means a 40% increase in loaded magazines.
The LSAT and other advanced designs are also not using the case as the seal - a shuttle door which contains the chamber pressure is the one item I haven't seen pictured on line. Proprietary I guess.
Sierra is loading the stainless duplex cases and selling ammo now, I like the concept for CCW carry, less to corrode. If I see those at the range I'm nabbing them.