If I recall, one of the gun YouTube channel tested plastic cased ammo and found them to be quite subpar. I think it was Military Arms Channel. In ways such that the downsides, while not detrimental for usage, were definitely not worth the benefits (if there were even any) to switch to using it. And I think he did mention they were also quite the expensive ammo.
The main benefits of plastic casings from what I've heard were better cooling and allowing higher pressures. That's why many of them were marked as having higher velocity than standard steel. I haven't watched the video yet but I guess both those didn't go as planned. I will watch it later.
Yes China switched to 7.62x51 NATO for infantry GPMG and (along with .338 Lapua) snipers. However vehicle mounted coaxial MGs and helicopters still fire 7.62x54R. That’s the case for both the PLAGF, PLA Airborne troops, and PLA Marine Corp. Interestingly, the three services also have a mixture of 30x165mm, 25mm, and 35mm shells for APCs, IFCs, and mobile anti-aircraft artilleries. Therefore, there is indeed a level of logistical challenges to be resolved in the coming years (such as retiring the 25mm and the 7.62x54R, and rechamber the coaxial MGs for 7.62x51 NATO).
Cooling yes, pressure not really. The pressure loading is mostly taken not by the casing but by the chamber. Most conventional brass or steel ammunition could easily be up pressured but doing so would come at other costs. Reliability, service life, recoil and damage to the weapon are all issues that occur.
The problem then is that it’s not backwards compatible with existing weapons nor industry. Which is why despite having been around as a technology since the 1970s it’s never been implemented.
