The cap was fixed to the contact gap with the gun (prolonged to form a "nipple") and ignited via the effects of the gun's "hammer". (The hammer is approximately similar to the cock uncovered on flintlocks other than that it doesn't clamp onto something.) In the situation of percussion caps https://handguns96047.blogdiloz.com/18372767/bolt-action-rifles-no-further-a-mystery