Stressing the cast bullets and later shearing them sounds like theory and complete lack of experience talking. Have you ever seen a 300 whisper barrel? Have you ever shot a cast bullet? Have you ever shot a cast bullet in a 300 Whisper?
Yes, I have not only seen, but shot a 300 whisper. It was an SSK barrel, first time I'd ever shot a contender. My shooting buddy has done alot of shooting with a 300 whisper a 338 whisper, and also a 50 American Eagle, a .512" bore on the 50 ae case, and he shot about 1/2 dozen different cast bullets out of it, and accuracy went south when velocties went much over 1000-1100 fps. All the whispers were SSK barrels and 1-8 twist.
As far as casting, I went through about 500#'s of wheelweights last year, and have 600#'s waiting to be turned into bullets. I sold or traded a goodly portion of those bullets, but shot 100-200#'s through handguns and rifles myself last year.
The shearing problem is well known with cast bullet shooters using fast twist barrels, and is the reason the mil surp swede mausers have a lower velocity limit then typical rifles 1600-1700 fps.
How about you?