It is impossible to push a bare lead bullet the length of the bore and then have it "grip the rifling". Its just not going to happen to any significant extent. Simple inerita will move them if the gun in bumped at the muzzle. Yeah, I tested this 25 years ago with Maxi Bullets. Recoil in a double rifle is far more severe. If the fit is loose enough they will move simply from the gun being carried muzzle down.
It has been repeatedly shown that recoil is increased and killing power is reduced in big bore round ball guns when a conical is used. You also need a fast twist to stabilise an elongated slug. I think the twist for your rifle is about 75". Far too slow for any solid base bullet. With a slow twist and a pointed bullet such as the 58 cal minie ball (72 twist was the US Army standard) the bullets seldom follow a straight path when striking anything.This has been noted by military doctors of the time and hunters using them on animals. Once the twist is significantly increased they will travel closer to line of sight. Thus is is possible to shoot an animal with a bullet with marginal stability and have it turn 45-to 90 degrees and miss the animals vitals all together.
If you take the time, smooth the bore, check the crown and try different powders and especially different patches, I beleive you will find a combination that will work as it should. I don't beleive conicals are the answer no matter what you may read.
I might also point out that at +- 1500 fps a hardened roundball of this caliber will completely penetrate a Indian Elephants head from side to side. This based on information in Forsythe's "The Sporting Rifle and its Projectiles". His heavy loads for the 14 bore (15ga ball) would do this and by using his tested trajectories he was getting about 1600fps.

Dan