Sentence:  <WSD>Snap caps</WSD> are dummies (no powder, no primer, no projectile) shaped like the ammunition for a particular gun. They're supposed to provide something for the firing pin to hit during dryfiring, so the gun's lockwork doesn't have to absorb this impact. It's a fine idea, but not all <WSD>snap caps</WSD> are made right. 
Meaning of the word marked with <WSD> tags:  A dummy round of (ammunition) used to check ammo feed and other mechanical problems.