A 1 in 20" twist is better for shooting conicals at long range, while a slow 1 in 59" to 1 on 70" is better for shooting round balls at long range.
The medium twist is a compromise but can still shoot shorter conicals and round balls fairly well depending on the barrel length and characteristics of the individual gun's rifling, etc...
You didn't mention the caliber of your gun, but the most popular saboted bullets in 50 caliber weigh around 240 - 250 grains and most any of them should provide a good starting point.
If it's a .45 then 180 - 200 grain bullets provide a starting point.
Shooting conical bullets presents a range of variables that requires more range testing to see what it likes the best.
Hornady has the new 350 grain .50 FTB bullet that is reported to have good accuracy in many types of guns, but it's not available in .45. They can found at Walmart.
http://www.hornady.com/story.php?s=770