For thou wilt set them as a butt. As the Hebrew word skm, shekem, which we have rendered a butt, properly signifies a shoulder, some understand it in that sense here, and explain the sentence thus: Their heads shall be smitten with heavy blows, so that having their bodies bended, their shoulders shall appear sticking out. According to these interpreters, the subjugation of the enemies of God is here metaphorically pointed out. But there is another explanation which is more generally received even among the Jewish expositors, namely, that God will shut them up in some corner, and there keep them from doing mischief; [1] and they take this view, because the Hebrew word skm, shekem, is often used to denote a corner, quarter, or place. As, however, the sacred writer, in the clause immediately following, represents God as furnished with a bow, ready to shoot his arrows directly in their faces, I have no doubt that, continuing his metaphor, he compares them to a butt, or mound of earth, on which it is customary to plant the mark which is aimed at, and thus the sense will flow very naturally thus: Lord, thou wilt make them as it were a butt against which to shoot thine arrows. [2] The great object which the Psalmist has in view is doubtless to teach us to exercise patience, until God, at the fit time, bring the ungodly to their end.

Footnotes

1 - Kimchi and others read, "Thou wilt put them into a corner;" which has been understood in this sense, "Thou wilt thrust them into a corner, and then direct thine arrows against their faces." -- See Poole's Synopsis Criticorum.

2 - This is the view taken by Ainsworth, Castellio, Cocceius, Diodati, Dathe, Horsley, and Fry. Horsley translates the verse thus:-- "Truly thou shalt make them a butt for thine arrows; Thou shalt take a steady aim against them." "I take," says he, "kvnk, [the word which he translates a steady aim,] to be a technical term of archery, to express the act of taking aim at a particular object." In our English version it is, Therefore thou shalt make them turn their backs." In defense of this sense of skm, shekem, see Merrick's Annotations. Gesenius takes the word in the same sense. Literally, "thy bow-string."

Therefore shalt thou make them turn their back - Margin, "Thou shalt set them as a butt." The word back also is rendered in the margin "shoulder." The word translated "therefore" means in this placer or, and the rendering "therefore" obscures the sense. The statement in this verse in connection with the previous verse, is, that they would not be able to "perform" or carry out their well-laid schemes, "for" or "because" God would make them turn the back; that is, he had vanquished them. They were going forward in the execution of their purposes, but God would interpose and turn them back, or compel them to "retreat." The word rendered "back" in this place - שׁכם shekem - means properly "shoulder," or, more strictly, the "shoulder-blades," that is, the part where these approach each other behind; and then the upper part of the back. It is not, therefore, incorrectly rendered by the phrase "thou shalt make them turn "the back."" The expression is equivalent to saying that they would be defeated or foiled in their plans and purposes.
When thou shalt make ready thine arrows upon thy strings - Compare the notes at Psalm 11:2. That is, when God should go forth against them, armed as a warrior.
Against the face of them - Against them; or, in their very front. He would meet them as they seemed to be marching on to certain conquest, and would defeat them. It would not be by a side-blow, or by skillful maneuver, or by turning their flank and attacking them in the rear. Truth meets error boldly, face to face, and is not afraid of a fair fight. In every such conflict error will ultimately yield; and whenever the wicked come openly into conflict with God, they must be compelled to turn and flee.

Therefore shalt thou make them turn their back - God can in a moment strike the most powerful and numerous army, even in the moment of victory, with panic; and then even the lame, the army which they had nearly routed, shall take the prey, and divide the spoil.
Against the face of them - Thou shalt cause them to turn their backs and fly, as if a volley of arrows had been discharged in their faces. This seems to be the sense of this difficult verse.

Therefore shalt thou make them (h) turn their back, [when] thou shalt make ready [thine arrows] upon thy strings against the face of them.
(h) As a mark to shoot at.

Therefore shall thou make them turn their back,.... Or flee and run away to private places, to hide themselves from the wrath of God and of the Lamb, though to no purpose; or "make them turn behind thy back": God will turn his back upon them, and be negligent and careless of them, and not regard them when they cry in their misery and destruction. Some Jewish interpreters (e) understand it of their being put together on one side, in one corner, and be separate from the people of God; to which sense the Targum inclines, rendering the word for "back" the "shoulder", which sometimes signifies unanimity and union, Zephaniah 3:9; and thus, being all together by themselves, the wrath of God shall be poured forth upon them, and they shall be destroyed at once: so the Christians were, by the providence of God, brought out of Jerusalem before its destruction; and the saints will be called out of Babylon before its fall; and the goats, the wicked, will be separated from the righteous, and set together at Christ's left hand; for they shall not stand in the congregation of the righteous: but the best sense of the words is, "thou shalt set them for a butt" or (f) "heap"; or, as it is in the Hebrew text, a shoulder; a butt to shoot at being so called, because it is earth heaped up like a shoulder; see Job 16:12; and to this agrees what follows:
when thou shalt make ready thine arrows upon thy strings against the face of them; that is, direct the arrows of his wrath and vengeance right against them; see Psalm 7:11.
(e) Kimchi & Ben Melech in loc. (f) "ponis eos metam", Cocceius; "humerum", Pagninus, Montanus, Gejerus; "velut tumulum", Junius & Tremellius, Piscator.

turn their back--literally, "place them [as to the] shoulder."
against the face of them--The shooting against their faces would cause them to turn their backs in flight.