National Army Selects New Cartridge in the .26 caliber size

[Ura-Ki]

Faster, Cheaper, Easier to set up manufacturing is likely the reason, weight may or may not be that big a difference!
Depending on the plastic, it may also be easier to recycle and cheaper! IDK, will find out as soon as they issue it!

 

[Stomper]

For the Heat issue, I believe it's a heat rejection thing, as in the heat is absorbed by the plastic as the round is fired and instead of the heat sink effect migrating to the chamber, it's carried out during ejection! Course this don't change the throat area and down the barrel as the flame front expands, but...............
IDK, but that's my best scientific explanation! Sort of how heat rejection coatings on internal engine parts help cooling, by reflecting heat, or rejecting it and expelling it out of the engine!

Plastic doesn't work that way as it's typically an insulator and not a
conductor and the "absorption" of heat is WAY slower than metal. What heats things up is the plasma/gases behind the projectile going down the barrel in the flash of a millisecond. The barrel gets heated as well as the chamber (but not as much as the barrel), but the barrel heat also gets conducted back to the chamber and receiver (via the metal barrel) because of thermal dynamics: heat always travels from the hottest areas to the cooler areas... because nature abhors a vacuum.


That's why you have to swap barrels on a machinegun and not just have a complete second gun. Have you ever fired a machinegun until the barrel gets red-hot? (Don't let the 1SG catch you doing it.... LOL)