I would start with a circle, and chisel out the spaces in between the points. You can pare off a little material on either side of a point to make them even.
The stars that are outlines take much less of a hammer stroke to impress into the metal. A star with a solid center needs a much bigger hit, and quite possibly, several hits to get it to the depth you like.
Be careful about stamping the barrel over the bore, you can dent the bore in. Stamp over the breechplug area if you can.
Tom