With cast bullets, size (ie..fit) is king. A poorly sized bullet is going to leave some of itself behind at a slower speed. Size your bullet .001 over groove diameter, start slow @ 10gr of pistol powder, Unique for example. And slowly work up to your most accurate load without signs of leading. My model 70 featherweight likes 12 grs. of Unique, very accurate inside 50 yds. No leading. I use 130grbullets from Badman Bullets. You can reach jacketed velocities with harder bullets but prob won't get much above 14-1600fps with straight wheel weights.
JD