Viking,
If you're shooting surplus or crap ammo, it's not a big deal, because you're probably not going to care how the brass is treated.
HK doesn't care, because one, you suck, and two, it's a combat weapons system that requires the unique chamber profile for reliability in the field. Likewise, soldiers don't police their brass to reload it after a firefight.
By way of explanation, the HK/CETME roller-locked delayed blowback action needs longitudinal chamber flutes to "float" the cartridge brass in a layer of combustion gas as a means of extraction assistance in that violent system.
Each fired round has multiple striations that go from case head to case mouth afterwards. While it's true they can be resized to *approximate* nominal dimensions, they are far from optimal, let alone looking very good.
The flutes in the brass can be ironed out, but never completely.
If you're gonna own a HK91 variant or CETME .308, it's a fact of life. The action won't function properly without some assistance from the chamber and barrel pressures via those flutes.
I've fired them, been offered some good deals on a few, and were I to own one, I'd probably get a bucket or three of surplus (thicker mil-spec) Lake City brass and reserve it specifically for that rifle.