What does this quote mean said by claudius in hamlet act 4 scene 1?

Claudius is telling Gertrude that he's going to do his best to cover up Hamlet's murder of Polonius. If he succeeds, he says, possibly (haply) he can avoid damage to the royal family's good name. Claudius says that slander, news or rumor or gossip that has the power to damage somebody's reputation, can travel an enormous distance (the world's diameter) with the same deadly accuracy as a cannonball fired at its target (As level as the cannon to his blank). But he hopes that in this case their family will be able to dodge the bullet, so to speak. He hopes that the metaphorical cannon shot will miss them and strike only empty air, which is incapable of being wounded.