Plasma weaponry is a staple of science fiction, but is often objected to on the basis that it expands too readily. There's no way to effectively deliver it to a target without spending a lot of effort of containment.

This is inspired by this question which describes two directed energy weapon systems, with the second one originally labeled plasma, then struck through and labeled ball lightning, silicon vapor theory.

What would be the mechanics of the actual firing? Bullets use a barrel, lasers use lenses, railguns use, well, rails. If I was building an infantry weapon that fires ball lightning, what would the the gun look like? How would it operate?