Abstract: The strongest constraints on sub-GeV feebly interacting particles are typically derived from beam dump facilities. Historically, these constraints rely on a conservative (under)estimate of the flux focussing only on production within the first interaction length. In this talk I will explain how to consistently include production from the subsequent electromagnetic cascade in the target. Sensitivity can be substantially enhanced at lower masses and weaker couplings (the ``lifetime frontier'') by including electromagnetic cascade production.