Here the original text has the definite article the in front of the second instance of “weapons of war”. The editors for the 1920 edition deleted this the (the change is marked in the committee copy). However, the reason the definite article is there in the original text is because the text is referring to the weapons of war mentioned at the beginning of the verse. The verse-initial clause (“Moroni had prepared his men with weapons of war”) means that Moroni’s men took extra weapons of war along with them (in addition to their own personal weapons). The text is not saying that Moroni had his men arm themselves, an obvious given. Thus the definite article later on in the verse (“and cast in the weapons of war in unto the prisoners”) refers to those extra weapons that Moroni’s men brought along.
Summary: Restore in Alma 55:16 the definite article the before the second instance of “weapons of war”; here the the refers to the earlier instance in the verse of “weapons of war”.