Sentence:  Therefore, the shotgun <WSD>primer</WSD> is quite self- contained. It consists of a separate metal cup called a battery cup that contains the <WSD>primer</WSD> itself. That's why it is considered a two-piece <WSD>primer</WSD>. 
Meaning of the word marked with <WSD> tags:  Any substance or device, such as priming wire or blasting cap, used to ignite gunpowder or other explosive.