Muskets or any flintlock firearms are generally always reloaded via the muzzle like a cannon. First, a ‘powder horn’ or ‘powder flask’ is used to pour ‘grains of shot’ aka gunpowder down the muzzle and into the bore where it settles at the breech. Experts may be able to eyeball how much powder’s needed for a successful ‘fire’, but many people use a ‘measuring flask’ in order to make sure the correct amount is inserted. For many later muskets, 100 grains of shot is recommended.