Let's sum up things:
A plasma rifle is a common sci-fi trope, that fills the role of the ultimate death machine, just like in real life, heating up the target, expanding at high speeds and releasing deadly x-rays that are so much cancer.
However, problem is that plasma dissipates too quickly, to hit the target.
Now, in this sci-fi setting, plasma rifles were essentially the by-product in the development path of the fusion technology and ironically, also powered by it.
The basic method is that a toroid shaped plasma is created, then accelerated toward the end of the rifle similarly to railguns, the projectile is self-confined. This method of operation was based on this where scientists were capable of producing a self-contained plasma doughnut, that lasted for ten milliseconds, with a few kilojoules.
What measures can I make to increase the plasma's lifetime with the least possible amounts of materials, that would need to be carried around and can I fuel the plasma further, with the railgun as it passes through the rails?