I shoot conicals out of my White with 1:24. It is extremely accurate.
To shoot lead accurately you do not need or want 130-140 grains powder, you score or melt the the side of the bullet and loose accuracy and velocity. Some guys use a over powder wad also. Try shooting lead conicals at 76-90 grains powder and you will probably be very pleased. Most M/L's will not burn 140gr powder in the barrel. Try laying a white sheet out in front of you about two feet and fire your rifle. If you find residue (unburned powder) on the sheet than you are not burning the charge in the barrel. Once the bullet leaves the barrel it don't care if the powder is still burning.