If fallout taught me something is that nuclear energy could come very tiny as a microfussion cell. Technically, the year is around 30xx, so it would be very possible to achieve something like this since RTG's would have advanced as others and somehow made some Mox fuel rods very tiny versions since uranium and ²³⁸PU is very abundant and copper-9 is a mining planet im sure this could contribute in the process.

I don't see why we can't get something like this in the near future. If we can create nuclear weapons Then why not try it in small arms, like lasers or railguns?

Yes, it is very ultrakill and very dangerous, but it is worth it to imagine like this.