This is a couple that should be glad, at least, that they are breaking up.
He says that she reminds him of a "gun." In trying to unpack that, she guesses he means that she is "lethal at close range" with her words, and also capable of shutting down communications (she has a "silencer") and shocking people.
She feels, however, that she is a "needle." She's always "pulling on the thread," which could either mean that he is as annoying as a loose thread... or that when she pulls on the thread of one of his lies, it unravels endlessly into an unbroken string of falsehoods. Also, he doesn't listen; she is always "making the same point" and "wondering if [he] heard."
Meanwhile, what weapon does she think he is? A "hammer." He's very "blunt" in his honesty. He's "heavy at the end," which seems to mean that when he starts to say something hurtful, he never stops before finishing. And he is "coming down on [her]" with criticism and threats from on high.
She then admits that she has a secret weapon. She likens it to a "pocket knife" in that the blade is "concealed." This language implies that she knows a secret of his that can hurt him. She doesn't want to use it, but she will if she is backed into a corner. How do we know it's his? Once he "sees" it, he will want it "back." Perhaps it is an incriminating photograph or receipt.
She concludes that: "If I am that weapon/ I am pointing now at you." What has been a name-calling contest has now escalated to threats.
Why did it get to this point? This is not just a stand-off. He has a "hostage." Evidently, the divorce involves a custody battle, and he's winning.
If he forces her hand, she will ruin his reputation. But if he backs down on the custody issue, she is willing to continue to negotiate: "We'll talk this down until we see this through."
This is a couple that needs to no longer be a couple. It is good that they are separating. Does it hurt? Yes, but if they stay together, they will just keep causing each other more pain.
I may not be a marriage counselor, but if a couple's fights are about what kind of "weapon" the other person is, they probably they should no longer share a mailbox.
Next Song: Harbor Song