What's happening here?

I haven't seen that in a long time. H&K used to have chambers that did that. Don't know if they still do or not.

HK91/PTR91 variants.
Fluted chambers.

Fluted chambers will not sick a piece of brass. These are semi auto rifles in .308 caliber. The chamber stays cooler and provides positive ejection.

Tinybob wrote:
What would be the purpose of a fluted chamber?

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Here's an article that explains it. https://en.wikipedia.org/wiki/Blowback_(firearms)#Delayed_blowback

The flutes allow some propellant gas to travel back into the chamber and "float" the cartridge case on a layer of gas to ease extraction.

The brass is generally safe to reload but becuase there is some deformation where the brass was supported by the chamber verus where it was supported by propellant gas, the cases may be more susceptible to neck splits at a lower number of reloadings than cases fired in conventional chambers.

That's exactly right. The cases are being extracted under much higher pressure than with a gas operated system, the fluted chamber allows the brass to be extracted by reducing the contact area between the chamber and the spent cartridge

Tinybob said:

So it's pretty much for high speed, high volume shooting?

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It's also for the premature destruction of brass!

When I was looking for a varmint rifle back in the 80s I had a salesman that was extremely knowledgable explain rifles to me. I had never owned a centerfire rifle before and knew nothing about them. He went over why the heavy barrel and the types of rifles. To this day I still remember that to explain things he had me handle an HK91 as well as other rifles. The balance of that rifle was amazing, but the thing that stood out was I noticed the flutes in the chamber. I remember he told me the purpose was to put a cushion of air between the case and chamber to make the case extract easier. Soon as I saw the pictures I remembered that lesson.

The HK rifles have no primary extraction. Just a straight back pull. Most other rifles twist the brass in the chamber before going back. The flutes seperate the brass from the chamber walls. Without the flutes the HK would be sticking brass in the chamber almost every shot.