My preferred rifling form is Micro Groove which has been lapped. The fine grooves deform the bullet very little, which leaves it in balance, while the large number of grooves leave a lot of gripping area for the rifling because there are so many edges.
Probably a larger diameter will do the trick, as perhaps the most important factor in bullet fitting for rifles is to get the chamber neck full so the bullet can't tip on takeoff.
You didn't mention anything about which powders you are using. To get the power up, you need slow powders same as for heavy loads with jacketed, not the fast ones listed in most cast bullet manuals. I've found W 748 and powders of close burning rate to produce the best results.
There are a couple other important factors.
To get speeds up to near full power, (and even exceed 30-30 jacketed levels) Bullet hardness must be around 12 to 14 bhn minimum, and at his hardness you can expect expansion on a par with the best jacketed bullets.
Did you use push through slugs to determine when you were done lapping. They will tell you whether you have the barrel straight and smooth. Polishing after lapping with lead is also mandatory, to smooth the barrel sufficiently. In other words, lapping cuts the barrel straight and true, and knocks off the rough edges that could cut up lead bullets. Polishing puts a slicker finish on the surfaces. This is all covered in detail in the lap instructions.
