There is a 6.5x55 improved.  I know the 6.5x55 is bigger in the head than a 6.5x257 improved, like you said, about the same as the 7.7 situation.  There are a couple versions of the 6.5x257 Imp, with various shoulder angles.   Perhaps differing amounts of case blow out too.  Someone could've done it, some guys just don't have better sense.

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