HK .30 caliber weapons are nothing to write home about. I do like the HK-21 series rear sights better than the rotating drum of the G3. It’s still pretty crude, but it works. The claw mounts are really heavy for what they provide, but hitting targets with 4X scopes at reasonable distances is easy.
HK rifles are nose heavy. The HK21’s are really nose heavy (and you can’t hold on to the barrel without BBQ’ing your fingers and the side grips introduced later on are a poor substitute for getting your elbow onto your chest), but pretty light compared to most GPMG’s. I think “poor” best describes HK ergonomics, although HK did eventually attempt to reduce weight with the introduction of polymer lowers. The increased size of the HK21 charging handles made them easier to operate compared to G3 handles. The HK21’s buffer and butt pad are a definite improvement over the thin butt of the G3 that refuses to stay in the shoulder pocket during firing. That’s fixable, but is indicative of poor design.
The HK’s primary advantage over its Belgian rival was and is cost. A HK21 or HK23 is faster and less costly to manufacture. I would have skipped steel and plastic ammo containers and gone right to fabric with plastic stiffeners sewn into the material and a steel mounting bracket.
A closed bolt roller locker would’ve encountered problems in US service. However, with a few mods (getting rid of the mounting brackets, plastic lower, no bipod, reworked barrel shroud that completely encloses the barrel) you could have a 16 pound belt-fed automatic rifle that would seriously lighten the load over what was available at the time.