Having shot a good many Speed Goats in eastern MT (terrain like Wyoming), I can tell you with authority that the SKS in 7.62x39mm, while not my first choice, would be a fine Antelope rifle and catridge to about 300yards. Yes, you are 14" low at that range, but given the gun's 6 O'clock style sights a backline hold at 300 yards will strike the vitals. I have a friend who uses one on mulies, and when he shoots they die. He won't shoot beyond 300yds, because not only is the bullet running out of killing power, 300 is the limit of his ability to keep his shots on a paper plate/the vitals.
Will the little 123gr bullet shoot diagonally through the hip bone to the far side shoulder? No. But even with a 7mm Mag 150gr neither he nor I would take such a shot. From the front, side, or quartering to, this bullet WILL kill any deer or antelope you hit correctly with it.
Before you try it though, be sure to practice that two-stage military trigger, as it is the key to shooting the SKS accuratly