Conicals are of no use in BP smoothbored guns. They require rifling to stabilize and a tumbling conical reduces needed penetration.
: The 'rifled' slugs sold today for 12'sa and other gauges, the Foster-type, do not spin in the air, but have grooves in the surface to reduce bore contact so they can be shot through any choke without damage. The grooves look good, but don't go all the way to the bottom of the slug, thus prevent the passage of air which in theory might cause the slug to spin. Recovered factory loaded rifled slugs are almost smooth on the outside, with little left of the rifling marks, and this testing was from a non-choked gun. The slugs, undersize at the start, obturate in the bore to fit it, become mch shorter in length and thus the 'lands' on the slug compress against the bore's wall. The obturation is due to the rapid shotgun powders used, and of course, due to the very soft nature, almost pure lead of the slug. This also goes for the Brenneke-type slugs in that there is virtually no spin in the air.
: Round balls should be the only projectiles shot in smooth bored BP guns and are what many of such guns were designed for. The exception of the "Cape Guns" which shot short conicals. These were spun by the short, but deeply rifled 'chokes' on the muzzle end of the tubes. I've always had a passion to rifle the chokes of a modern 12 bore to see the effect, if any. In these, a patched or cose fitting round ball would be the easiest to get to spin enough to stabilize as the round ball requires less spin than any slug.
: Rifled slugs do gain a modicum of accuracy due to the shuttle-cock effect of the hollow base. The hollow base concentrates the slug's weight at the nose, causing the slug's rear to not try and pass the nose in flight. They apear to tip, straighten, tip, straighten, etc, etc. This works fairly well, but rarely do they shoot better than a proper fitting round ball, as proven in my tests and those of my friends. Too, this almost tumbling slug, will lack the penetrative power of the heavier, round ball.