Douglas barrels are marked at the muzzle with caliber, and twist rate. But, as mentioned, they did make special barrels for other companies that had different twists, and weren’t marked on the muzzle.
Cast bullets cast from lead alloyed with other metals will be a bit larger than one cast from pure lead, so they will be harder to load.
Voids in cast bullet is an old wives tale as related to accuracy, perpetuated by the companies that make swedged bullets. An old boy that had been shooting muzzleloaders since his childhood, regularly took young nimrods rejected bullets, and shot rings around them with their own rejects ( me being one of those nimrods).
The point of a GAIN twist is that you can shoot heavier charges than a fast twist. IMO, shooting a GAIN twist with the same charge as your fast twist is a waste of an expensive barrel.
I have heard good accuracy can be had from a slow twist .40 cal. But the slow twist is more finicky about the charge, and the patching.
Hungry Horse