Implement for packing down the charge in certain guns (6)
I believe the answer is:
ramrod
(Other definitions for ramrod that I've seen before include "Pole for pushing charge into a gun barrel" , "It's used to push a charge into a firearm" , "Device for charging gun, proverbially stiff" , "Martinet" , "Stern and inflexible person - part of a musket's equipment" .)