Okay, so a coworker (who did GRIM’s effects) and I looked through some of his kit, and I think we found what you were seeing. On Paragon, one of the sparks that got used around was called “hotdog” and most of GRIM’s stuff used that. The basic idea is that you switch to a stretched version of the spark texture as you go faster. It doesn’t actually do any gradient stuff to fade the trail toward the back, but it would be easy to add another lerp to handle that too (I think the version in paragon just looks like it has a gradient on it due to TAA and general persistence of vision).
This particular material actually works slightly differently than I said before since what it actually does is stretch the middle of the texture out based off of the proportion of the X size of the particle to the Y size of the particle (those nodes are farther to the left than this screenshot), but the idea is still very similar. The default value for “max speed” is 200, but I think the effect of it is much easier to see if you set it to 2000. (I think other characters might have had derivations from this material, but this is the basic idea. If you want to find them, look for “hotdog” as that describes the idea of stretching out a spark so that instead of looking like an ellipse when stretched, only the middle gets stretched like this:
⊂⊃ vs o
Regarding the motion, we really didn’t have a lot special going on. GRIM’s muzzle flash is just a Sphere emission shape (with velocity on) + Initial location (that is a stretched out line in front of the barrel) + a bunch of random initial velocity forward. Lots of other characters would have had vector fields to add some of that swirling motion, but GRIM at least doesn’t use much. You probably could do that UV offset/warp stuff I was talking about before too, but you’d want to shrink the texture on the quad in order to have some room to shift without going off the edge.
Hope that helps clarify!