The hollow rammer forms the nozzle from the whistle composition (no clay or other non-whistle comp nozzle is used in whistle rockets).
It is also used to ram the whistle composition up to the top of the spindle. The solid rammer is used to ram the remainder of the fuel above the spindle.
The spindle is made from stainless steel which resists galling and erosion from grog, titanium, and other hard materials.
All rammers are knurled to make them easier to extract from the rocket tubes.