What follows is advice from C.E. Harris, which he wrote in 1994 (I've edited out all the stuff not relating to 7.62x39).
C.E. Harris-
I have reloaded thousands of rounds of 7.62X39 ammunition with both cast bullets and jacketed, in the AK and the SKS. I got frustrated with the AK as accuracy of these seems is generally poor, at best about 5-6" at 100 yards. The SKS is more reasonable, about 3-4" and is still a real bargain these days. Most SKS rifles group better with cast bullets than they do with ball ammunition. If you are new to cast bullet shooting or not inclined to do a lot of experimenting, I recommend 14.5-15.5 grs. of Hercules #2400 with the Lee .312-155-2R bullet, cast of wheel weights and sized .312". With this load the rifle functions like the proverbial pony trotting, and dumps the cases right at your feet! You can also try 18-21 grs. of 4198 or 16-18 grs. of 4227 with any bullet from 122-125 grs.
The gas ports on these guns are larger than they need to be for reliable functioning. Lead fouling of the gas port is not a problem, but the gas piston and cylinder will foul, which can affect functioning if neglected. The gas piston should be removed and cleaned with Break Free, Marvel Mystery Oil or Dexron IIe ATF and 000 steel wool after each use, and the gas cylinder scrubbed with a shotgun bore brush. Liberally lubricating with Break Free, Marvel Mystery Oil or Dexron prior to reassembly eases the fouling problem.
Above 1800 f.p.s. I use the same alloy, but cast the bullets "hot" until they are uniformly frosted, then quench them directly from the mould. The heattreated, double-lubed bullets will stand 2000 fps. in wheelweight alloy without leading, if shot straight from the mould without sizing, being GC'ed in a .313" die. A caseful (28 grs.) of Accurate 2230, 2460, or H335 gives around 2000 f.p.s. in the 7.62x39.
Accurate Arms 1680 is very similar to the Olin 680 Ball powder, though slightly slower. Both of these powders are used in 7.62x39 ball ammunition, with a charge of 24 grs. being correct for service velocities with 123-gr. jacketed bullets using the Accurate-IMI propellant, and 23 grs. for the slightly faster Olin powder. For a full-power load approximating service ammunition you can also use 26.5 grs. of RL-7 or 25.0 grs. of IMR or H4198. With cast bullets the minimum load with 1680 which functions the SKS with the Lee .312-155-2R is 16 grs. In the SKS this produces about 1420 f.p.s. and 3" groups at 100 yds.; 18 grs. gives 1630 f.p.s. and 4" groups in the SKS. I did two strings with the Olin 680 for comparison with the NEI 155-gr. cast bullet at 18 and 19 grs., respectively, which gave 1810 and 1930 f.p.s. I would not exceed 19 grs. of Olin 680, or 20 grs. of Accurate 1680 with the 155-gr. cast bullet, as these are both full loads.
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I hope that is helpful.
Jimro