It is full-auto. Its statistics make it act a lot like an assault rifle rather than a rifle. Compared to the assault rifles, it is overall statistically the best, having the lowest recoil, the same magazine capacity as them, a fast reload speed, good iron sights and a good rate of fire. Comparing it to the rifles, it is again statistically the best overall, with a superior magazine size to both rifles, it is full-auto compared to the bolt-action and semi-auto firemodes of the other rifles, and it has lower recoil than either of the other rifles (though recoil doesn't really matter on the M1891/59 since it resettles before the next shot).
Overall, although there are better choices, the SKS isn't a bad weapon to end up with.
In both games, when the player fires, the animation glitches, as the weapon is held further away and the player can see their arms floating a bit, disconnected from the player's body. This does not affect gameplay. This can be fixed by reloading or changing weapons.
When the player pulls back the charging handle, they do not actually pull anything, as the model lacks a handle.