Well, you could always pair a contact explosive with an oven timer (y'know, one that vibrates when the time runs out). With good funding you could use a cell phone set to vibrate, though that would add traceability through call logs.

If you can't get a contact explosive, just pull the bullet out of its shell, sprinkle some thermite in, and boom, should work as an ignition. Just rig a way to pull the trigger and you've got yourself a boom.