Well yeah, you used a cup and core bullet at magnum speeds (one that's known to be on the explosive side no less)... That's asking for a shallow wound and a blown up bullet...
With the sks they would be going much slower, where they will be far less likely to fail miserably. Now, I wouldn't pick the sst either due to its tendency to come apart (not to mention the poor sectional density of a 123gr 30cal pill) but there aren't a lot of other options for 310cal bullets. A 311cal 150gr soft point (such as what PRVI makes for 303) would probably make good medicine though assuming you keep range short enough to keep velocity high enough to expand.