I would say in all most cases energy stored in matter by some means. For nuclear bomb, nuclear energy stored in uranium.

If we fall back to common weapons (firearms are some kind of explosion) then we could say the energy of shot is held in powder and this is a chemically stored energy. To create plasma you have to spend some energy. But this is a point of explanation (or just handwaving) you could do. It's hard to imagine how (and from where) you could receive energy to heat up somewhat to plasma in fraction of a second. Ammo storageĪs described above you can't hold armed ammoes for hours (in you weapon or extra clip) -> you need to create plasma just before shot. You can't cover the plasma by any hull in small size - it would be melted.

Think of plasma as extremely hot (thousands and millions degrees) gaze.

So in the real world it's impossible to generate such strong magnetic field in bullet size. As far as I know, it's very difficult to keep plasma in some seconds: you need strong magnetic fields which generated by current of kiloampers which requied very thick superconductors. Today, the most developed project about keeping plasma in limited space is ITER. A bullet with plasma inside is not possible.

Xcom 2 plasma rifle