Sorry if this is a dumb question but I'm really not an advanced Blender user at all, but, when creating and positioning the circle emitter, would it also be feasible/easier to just go in to the gun model itself, select one of the edge loops in the barrel, duplicate and separate it from that model as its own object, and then fill it in? That way it'd be aligned and scaled with the barrel perfectly? Anyway, thanks for all of your amazing tutorials. I am honestly absolutely loving them. :)
Your suggestion is a much more reliable solution for sure! Great thinking :)