I posted this on Accuratereloading.com on the same post on using filler and blowing up his Krieghoff barrels. I don’t think he used enough filler to take up all the space and got the results G Wright talked about the base of the bullet acted like a secondary obstruction in the chamber/ barrel.
Hear is my other post.

I have read this thread and found it quite interesting. I read up on this phenomena back when I was starting to reload as I was using a filler over the powder in my old 45-70 trapdoor and was afraid of damaging it. Charlie Dell’s work was helpful in understanding the problem. A problem I didn’t know existed until a friend cautioned me about it.
I think the problem is caused by the pressure front caused by the rapidly expanding gasses of the burning powder. With a over powder wad this becomes a part of the pressure wave hitting the back of the seated bullet resulting in the barrel obstruction syndrome. This is why filling ALL THE SPACE is important. This retards the pressure wave enough that you don’t have a problem. Uniform ignition is just a happy by product of doing this.
“Federal factory 470 ammo sometimes has large variation in velocity due to the position of powder in the case when the trigger is pulled.”
I wonder what would happen if you pulled some of these and used a foam filler as Kynoch does and see if it helps.
Bill[