Slug weight, twist rate and fps?

Use this bullet stability calculator and you will know where is a good starting point for your ammo and barrel. Keep in mind the polygon and fx barrel’s twist rate isn’t always absolute thanks to the smooth lands so try to get stability factor of minimum 2.0-2.5 based on their published numbers.



https://bergerbullets.com/twist-rate-calculator/

Not so much the weight of the projectile. Though somewhat related to each other, it's the projectile length that will be the main determination of the twist rate for a given caliber (as far as stability).

Exactly. 

Similarly, I am shooting my own hand cast HP's at normally 30's to 960's and have fantastic accuracy. But only after I removed the choke. it was terrible before. 

Fx slugs crush down similar to a pellet, so a choke works.However With normal slugs, chokes cannot help, but can really hinger accuracy. ot to mention lead up much more rapidly.

Why, because you either slug the bore and size to the groove dia which is not correct for the choke,or you size to the choke and the slug is not stable traveling down the bore until it hits the choke. And can very well be skewed when it enters the choke. Not good for accuracy. I will work some times, but not often. Sorta like Blind Hog Finds Acorn.