I could use them for larger game though
What do you mean by larger game?
As already stated the 150gr .310-.311 bullets are to hard for the slow velocities generated by the X39. If you must have a 150gr I would start with this one from Speer.
http://www.midwayusa.com/viewProduct/?productNumber=282179
In my Mini-30 for 150gr I use Hornady 150gr RN 30-30 bullets. These will have better terminal performance, bu they are .308 and may not be very accurate in your SKS.
For bullets of 135gr and up, my Ruger likes H-322, H-4198 and N-130. AA 1680 is a good powder for the x39, but I haven't had it do very well with bullets over 130gr.