They go off when they strike the target, that's good enough for me. Take a look at the link I provided for the cut away look inside...
I on the other hand was using them as targets with my Red Rider BB gun and the primer DID go off...no surprise now to us...from about 6 yards...
On the other hand I can give you testimony of one .44 magnum that exploded when it was dropped to a concrete floor...hit a nubbin of some kind I guess... I can go on...