The preferred place to smite someone in Second Samuel

Abner killed Asahel by smiting him uder the fifth rib.

Howbeit he refused to turn aside: wherefore Abner with the hinder end of the spear smote him under the fifth rib, that the spear came out behind him; and he fell down there, and died in the same place: and it came to pass, that as many as came to the place where Asahel fell down and died stood still. 2 Samuel 2:23

Joab killed Abner by smiting him under the fifth rib.

And when Abner was returned to Hebron, Joab took him aside in the gate to speak with him quietly, and smote him there under the fifth rib, that he died, for the blood of Asahel his brother. 2 Samuel 3:27

Baanah and Rechab smote Ishbosheth under the fifth rib.

And they came thither into the midst of the house, as though they would have fetched wheat; and they smote him under the fifth rib: and Rechab and Baanah his brother escaped. 2 Samuel 4:6

Joab killed Amasa by smiting him in the fifth rib.

But Amasa took no heed to the sword that was in Joab's hand: so he smote him therewith in the fifth rib, and shed out his bowels to the ground, and struck him not again; and he died. So Joab and Abishai his brother pursued after Sheba the son of Bichri. 2 Samuel 20:10