I think the reason you see this is most of the inline rifles on the market are developed with faster twists in mind to take advantage of the conical and sabot market. Most of the very small caliber such as the .32 & .36 are more a roundball type barrel and therefore are more common place among the traditional rifles. If you want to shoot a .36 caliber level projectile out of an inline, get a .45 caliber and load a .357 projectile. That's about as close as you might get.

I did hear once that Knight was thinking of making a small caliber inline but I never heard any more about it since.. Try a traditional rifle if you have a chance. You might find a whole new passion in muzzleloaders.