i have also to bad experiences, one one target and the other one on the scapula of a "Sanglier" with 75 grs steel insert
I asked the question to the members of the french bowhunters forum and apparently i'm not alone.
So this afternoon i tried to break a 75 and a 125 just to see what happen
I' can't wright it was easy but for the 75 one it's was not difficult .......
the result in pictures , i don't know if it is significant but if you have some ideas ...
exept one scapula Wild boar i had only good results with this steel adaptator for 5 big game animals since i 'm using it
75 on the left // 125 on the right