The containment provided by the pressure cooker means that the energy from the explosion is confined until the pressure cooker itself explodes, creating a relatively large explosion using low explosives and generating potentially lethal shrapnel. - https://en.wikipedia.org/wiki/Pressure_cooker_bomb
Correct. The Boston Bombers used the gunpowder out of FIREWORKS, which is relatively slow burning and not very high powered.
Hold a firecracker in your open palm and light it. It will sting a while and probably be slightly burned, but you'll be OK. Light it and close your palm tightly, you'll probably loose at least one finger.