So, you have some information missing from your descriptions.
1) Chambering - Which, from your tagline, I presume is 458 Socom
2) Method of coating your bullets - I am going to presume these are powder coated, vs Hi-Tek (2 part epoxy) coating
3) Whether these are commercially purchased or your own home cast bullets

I'm going to presume that you made the bullets yourself, since you seem to know the approximate hardness. Final (after coating) diameter of 458 Socom bullets should be .459 to properly obturate the back ends of the bullets (same as with regular cast). That's .001" larger than the theoretical bore diameter. But, you didn't indicate if that .457 was before or after coating. If it was .457 fter coating, then one of the most obvious issues is the bullets being undersized.

I am also guessing that you did the home baked powder coating (vs the 2 part epoxy - Hi-Tek) coating. The powder coating, in my long experience is too brittle to handle any mistakes in bullet sizing. Also, your hardness at BHN 11 is too soft for 458 Socom speeds. The back end of the bullets, even though not properly sized, and using the wrong coating, should actually be at like 20 or so.

So, what's likely happening is that because of the damage to the bullet when it sees firing pressures, you are ripping off the back end of the coating.

Solutions:
1) Switch to the Hi-Tek version of coating (hope that you haven't made too deep an investment in powder).
2) Cast the bullets in 100% wheel weight alloy, and water drop them from the mold. That will yield about 20 BHN
3) Make sure the final size is about .459
4) Don't over flare the cases. That will yield shorter case life. And, will grab the bullets so tightly (when you crimp) that you'll likely cut through the coating (especially powder coating). Flare only enough so that the bullet will slide in without shaving the bullet on the way in.