Hi Ed. By short bullet, I suspect you are talking about an ounce @ 437gr. or thereabouts?

I would go with something in the 36" range, maybe 40". It would still shoot round ball loads if you desired.

The .50 cal. Sharps usually had 36" twists and handled bullets quite long at black powder speeds, in fact, up to 550gr.

That is why I suggested 40". I suspect a 48" twist would also do well with either short slug and better than 40" twist with round ball.

I had some fair results using undersized round balls patched in .030" denim, loaded in 16 bore brass with 82gr. 2F loads - go figure.

I should note, the 12 bore rifled tube on my Mossy shoots round balls very well at 1,600fps and it is a 36" twist, so there seems to be a lot of leeway.