I’m guessing it’s more about the standard round that it fires. 5.56 or .223 rounds are built more for penetrating materials so when they’re up against a fleshy target, unless you hit them right in the vitals, might not cleanly kill and cause prolonged suffering. That’s not to say that the gun can’t be chambered in something that’s more useful for hunting but having a 20-30 round magazine for hunting is still a bit overkill.
The physical battery idea has been a thing for decades in the form of a pump storage plant where during times of excess electricity, they pump water up a hill, and when power is needed it works like a hydroelectric power plant. The problems with these however is that in order to get a meaningful amount of power and longevity, you need a lot of water and space to build one of these which makes them massive and expensive up front. I have one near me, but I also live near one of the biggest lakes in the world, which helps.