The cards sent to the Graveyard after the activation of “Full Salvo” are sent to the Graveyard by the effect of “Full Salvo”. They are not a cost. So they are sent when “Full Salvo” resolves, not when it is activated.
If you send a “Dark World” monster to the Graveyard for the effect of “Full Salvo”, its effects do not activate because it was not “discarded”.