houseabsolute is right on this. You do not see any blasting caps, or explosive devices, military grade or not, made with PETN, that do not have a trigger. Fancy-named explosives like C-4 get all the glory in the movies, but tricky bits like the triggers don't get any attention.

Thankfully for us, the same properties that make a trigger a trigger also make triggers kind of a pain in the ass to manufacture. If it will blow up easily on the plane, it'll blow up even more easily when you're trying to make it from scratch or if you're prying it out of an artillery shell (how in God's name one would try to attempt the latter, I have no idea).