Whilst The French “Amorce” means Primer in modern Translation, what you have shown are “Caps” ( Musket top-hat type); the English term “Caps” can also mean (Cartridge) Primers as well; such are the vagaries of the English language vis-a-vis (Nice French-derived term) some of the European languages.
Whilst “Amorce” can be used for both types of priming device (Musket and cartridge case), in English, “primer” is nowadays restricted to Cartridge cases, whilst Muskets use “caps”; but “caps” can also be applied to cartridge cases.
I suppose the “cap” because it was like a “chapeau”, especially the “top hat” style of large Musket Cap.
Just “un jeu de mots a’ l’anglais”
thanks for the explanations