I think you've got some good advise here. Pay attention to the chamber throats size & DO be sure the slugs will drop through. Additional taper to the forcing cone might not hurt but really should not be necessary.

I used to be in the business of casting & selling machine cast bullets. Frankly, commercial cast bullets can not be very hard cast & still commercially saleable because of cost.
The liquid bullet lube came in after I shut down my business but I have heard of folks who swear by over lubing with the liquid right over the commercial lubed bullet product.

True "hard cast" can be driven to very high velocity. Such speciality bullets, hardness tested, would not lead significantly. These are the bullets that Keith & others worked with.

My suggestion is to find someone with a proven load that does lead unduely in their guns... and try it in yours to determine if it's the gun or load. Since both your guns do this, I suspect it maybe the load or something about your proceedure.

Best Wishes,
Russ