There you go, exactly right. The faster twists of the new inlines require a longer, heavier bullet, or pretty light charges for target work. If the reasoning for trying a different projectile is reasonable grouping, possibly a different powder, Pyrodex, Goex2F or 3F, or some other powder with a different burning rate will give you what you are looking for. Example: Elaines Thunderhawk was only capable of 3 inch groups at 50 yds with 2F, but Pyrodex select under a 250gr SST grouped 1 1/2inches at 100 yds.You can shoot round ball from an inline, BUT there's a few tricks to getting it to work.
You'll need to know the twist rate. Round ball doesn't like to spin too fast and group well at the same time.
You'll find that if you drop your charge way down, you can produce good groups with round ball from a faster twist barrel... How far down will depend on your barrel.
A buddy of mine shoots round ball from an old, sorely neglected Optima Pro that he salvaged... hunts with it, loads PRB over 60gr of 3f. Groups about 3" at 100 yards but he won't shoot a deer past 75 with it. If he takes it up to 70gr, the "groups" are 8-12" and just get crazy above that. Serious curveball effect from 1:28 barrels.
One other concern from newer inlines is the their rifling depth. Most were designed with sabots in mind,and might not have rifling cut deep enough to engage well on prb..
This could actually work to your advantage in a fast twist barrel, as it might not "over spin" the ball... but I wouldn't count on this effect. It would be luck if it worked in your favor.
The buffalo bullets would give you better results than round ball, heavier/longer the better.
If we're talking the Buffalo Bullets (brand name) "Ball-ETS", they come pre-lubed...Using buffalo bullets are they lubed or plan or with a patch or cup ?