The remnant will be the instruments of Divine retribution against the gentiles. It will be the remnant's "hand" which "shall be lifted up upon thine adversaries." And it will be "all [the remnant's] enemies [which] shall be cut off." So, who will be the remnant's "adversaries?" Who will be their "enemies?"
The Lord promises to "gather my people together" --and the only ones He has called His people are the Nephite audience, never the gentiles. (See 3 Nephi 16: 8-9; 3 Nephi 20: 15, 27; 3 Nephi 21: 2.) The Lord's people to be gathered, the promised inheritors of the land, the chosen and covenant people are the remnant. This prophecy is about them. The gentiles are only included to the extent that a few of them will repent. (3 Nephi 16: 13; 3 Nephi 21: 6.)
The "sheaves into the floor" is a harvest image. It is an end-of-times view, because it involves harvest time. "Gathering the sheaves into the floor" is a reference to latter-day Zion, where a group is first "gathered" before the burning of the fields that always follows.
Again the Lord calls the remnant "my people" while clarifying that His people are those "with whom the Father hath covenanted." To covenant with the Father is to receive a Father. The Father does not covenant with strangers. His covenants are with His household. So this is the Family of God.
The "iron horn" and the "brass hoofs" are also symbolic images. What does a "horn" represent? In the context of judgment, does the "horn" hold additional meaning? Why is the horn said to be "iron?" What do the hoofs represent? In the context of judgment do the "hoofs" have additional meaning? Why are they "brass?" How stern and unrelenting will the judgment be? How complete will it become for the "people" to be "beat in pieces?" How terrible will the pouring out of judgment become?
Why would judgment be so severe upon a people who claim to be godly? Think about the introduction to Joseph Smith at the time the restoration of all things was offered. (JS-H 1: 19.) Compare that to the statement made by the mortal Christ when the Pharisees were confronting Him about violating the rituals and practices of the religious hierarchy at the time. (Mark 7: 5-9.) Christ offered them the fullness of His Gospel and they rejected it. The judgment which followed was unlike anything that went before. Christ warned them it would be so. (Matt. 24: 21.) Nevertheless, they refused to accept the fullness offered them, continued on in their religious traditions, and were besieged by Roman legions and slain en masse. The account from Josephus is difficult and shocking to read. Mothers cannibalizing their infants to satiate their hunger pains. It is as if hell itself opened upon Jerusalem.
Rejection of the fullness of Christ's Gospel carries terrible consequences. We have seen it before. And, when it was rejected before, it was done in preference to traditions from men. The arm of flesh and a religion multiplied the commandments of men until every aspect of life was controlled by religion. How one dressed, what they ate, how they observed the Sabbath, what things were considered clean and unclean, how to appear in public in order to conform to the right look, vocabulary and conduct. These were very religious people. I've discussed them in Come, Let Us Adore Him. I assume you're familiar with that.
Thank goodness we are not like them. We have the fullness, don't we? We are safe and in the right path and none can molest us or make us afraid. For we are the chosen people.
Well those other people (not us) who rejected the fullness of the Gospel, those are the ones who will be broken into pieces. Then their "gain" and their "substance" will be consecrated to the Lord. So they will live the law of consecration after all! Only it will be postmortem. That is, once killed, the Lord can use their gain and substance to provide for His people.
The Lord is, after all, the same yesterday, today and tomorrow. When He invites us in meekness to come to Him, we should realize that failure to come risks the judgments that have always been terrible to bear. Those Old Testament events we have a difficult time associating with Christ will become associated with Christ again. Just as His New Testament judgments were His and terrible to behold. He is the same. We should expect that when the time ends and we have not met our appointment, we actually do risk rejection and judgment.