I would remove the red dot and shoot a couple of 5 round groups at 25 yards using the irons. Do it from sand bags, not the bipod, as bipods like that can really screw with your accuracy. If you can keep consistent groups at 25 yards, move out to 50 and try again.
I suspect that scope mount may be loose enough to cause problems, as well as the bipod. In addition, every SKS I have ever shot has had a HORRIBLE trigger, with a long, heavy, creepy pull. It's almost impossible to get decent accuracy with a trigger like that, as you need to be rock steady in your hold and put a lot of effort into keeping it on target while waiting for the break.
Finally, keep in mind that cheap 7.62x39 ammo is not known for being very accurate.
With decent ammo, solidly mounted sights and good trigger control, though, any SKS should be capable of 4 MOA at 100 yards.