I think the shattering problem is related primarily to antimony content rather than just to what you harden it to. That is, a linotype bullet will shatter more readily than a low antimony alloy bullet heat treated to the same hardness. I know Veral Smith said wheelweight alloy would shatter on bone in Idaho's extremely cold weather. The rules of thumb I've heard in several places is not to exceed 2% antimony and not to let the tin exceed the antimony level or hardness will age away more rapidly. The 50/50 alloy would be about there (and a little tin could be safely added to it for easier mold fill). Beartooth Bullets, for example, and Randy Garrett's cast bullets also, IIRC, are heat treated to a BHN in the low 20's, but being low antimony alloys, don't shatter.
The issue of penetration seems to depend on how wide the meplat on the bullet is. With the wide meplat designs, like Michael's, the wound channels are often surprisingly large and long, both, even if the bullets don't deform. In that instance an exit hole is just helpful as a second blood leak.
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