The XM8

[Excalibur]

The SCAR and the ACR have survived extreme temp tests, but I believe the SCAR was picked because it was cheaper and lighter than the ACR, which was made heavier from the Masada.

Both the SCAR and the newer ACR have a metal upper receiver where the action is so it isn't like they are going to "melt" The XM8 is more of a spin off of the G36, so it's almost entirely "plastic".

I've seen MG36s run hundreds of rounds without melting.

In the end, it's mostly costs and lack of great improvements that is why the military isn't switching to newer rifles. My gripe with the SCAR is the reciprocating charging handle.

Originally Posted by Mazryonh

Anyway, while it's in service with a few small forces, parts of it have survived elsewhere, such as the related HK416 series of rifles. I wonder if the USMC-adopted M27 IAR's role could have been filled by the XM8 LMG variant.

I think the Marines adopting the M27 IAR is to go back to a lighter weapon system similar to the BAR, though I think the unofficial reason is for costs. The M27 costs less than the SAWs they are replacing, though for an "automatic rifle" role I would think they would play with the idea of larger capacity mags than standard 30 rounds.