Cast bullets don't do well with full power powder charges.
If you have Unique, Red Dot, or 2400 on hand you can work up some mild loads with good accuracy. If you have H4895 you can probably work up to 34 grains or so with good accuracy.
What you can't do is shoot them fast, the Mosin Nagant has a relatively tight twist, around 1:9.4 inches, this means that your bullets can actually deform from being shot too fast (which spins them too fast). 1,800 fps is generally the upper limit for Mosins and Mausers with normal lead bullet techniques.
Secondly, full power jacketed bullet loads smack the heck out of a cast bullet, so it gets jammed hard and easily deformed going into the barrel. Odds are you have some significant leading in your bore based on the loads you shot.
Now there are exceptions to this, but that's a bit more advanced than "buy bullet, load bullet, shoot bullet."
Jimro
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