One question; I see you have used Brennekes, how much velocity will they have with your loads? I ask because I've heard that the lead of the brennekes ist too soft to push it over 1700 fp/s, the air friction will tear the projectile apart. With pure lead it would start about 1400 fp/s.
And if the lead slug withstands the air friction it will fall apart if it hits a target because the lead will become frangible if it hits a soft medium at those speeds.
Watching the whole project with great interest! Those 1015 grain slugs look pretty entertaining!