Or, y'know close-range stabbing.
Here is about the point where I say: "Screw physics, I don't know enough about them and they're getting in the way of stuff."
The problem here is that I want my sci-fi part to be as hard sci-fi as possible. Plasma isn't really a great weapon for a long time.
It would make an excellent starship weapon. The self-sustaining magnetic bottle + Relativistic velocities combined with the near vacuum of space would permit the projectiles to be quite long lived. Normal magnetic deflectors would be designed for low density cool plasmas found in space-- Not this kind of insane stuff. You would need to be able to survive flying through a star to deflect this kind of munition.
It would rip a really big hole through just about any kind of armor imaginable. With a starship, the expense budget is already going to be absurd. Power requirements for FTL travel will be absurd-- so the ship powerplant should be sufficient to drive such a weapon.
Fired from orbit down onto a planet, with the projectile traveling at a significant fraction of C, it would reach the ground before dispersing completely.
The problem in orbit is cooling. This weapon will make a lot of heat, and will be very slow to fire. It would pretty much quell any ground engagement that goes awry, and needs asset denial.
(At relativistic velocities, the atmosphere between the ship and the ground becomes added fuel for fusion reactions as the dense plasma plows its way past. Imagine a "reverse mushroom cloud" blasting down onto the surface.)