Oh you can build one out of literal trash from the 90s.

It's just... So there's a lot of energy in gunpowder. Like, a lot. And even being super inefficient and losing a ton if that energy, it's really good at pushing stuff. Its just naturally good at exploding, and over the years, we occasionally make it even better at exploding.

And we have some pretty good batteries but they really dont compare.

And then, you know, you need to release a shit ton of energy all at once right? And batteries aren't very good at that. Even if you make them be explosives instead of batteries, they still aren't anywhere near as good as gunpowder at that.

So you also need a capacitor. Which is more stuff.

And you've still got a really weak gun, and you need to charge it between shots-not just the battery, but the capacitors(?) In a way you can't really just rotate out-notice how there are three sections of wire there? Thats three thingies to accelerate the slug, which each need a quick (and super well timed but thats not usually a problem AFAIK) boop from a capacitor. So you can't fire fast for long even if youre carrying a huge battery. And overheating is a huuuge problem, because remember what heat does to conductivity. They are not friends.

So you can't use it in a sustained fight you can't (as easily) hurt someone armored and you can't make your dick feel big with it. Which are most of what militaries care about.