.250/300 is a wonderful round - ballistics well above what it's appearance would suggest. I've wanted one for years, but incredibly, never put one together.
Good as the standard case is, the AckImp case is that much better.

Another great round is the 6mm International, literally a necked down .250/3000. Another 'want' of mine and only some 50fps behind the much larger .243 in average ballistics in 22" bls.