I’m trying to make sure I understand your question correctly;
You’ve used a spline mesh as a projectile and now you’re trying to add a particle system to it as well?
Firstly, I didn’t think you could add a spline mesh to a projectile, so not sure how you got that working. lol. I thought splines were static only, but maybe not?
Next, you’re trying to have a particle that starts at one point on the spline and ends at another point in the spline? As far as I know, you need a beam emitter for that with a Start and End point and then you can set them in a blueprint.
Here’s an example:
