The no 2 was designed because the straight walled 3-1/4" was supposedly causing malfunctions when the cases expanded during firing and subsequently failed to extract or became jammed. Explanations I've read cite the reason being due to the heat of Africa/India causing higher than expected chamber pressure, and so the No 2 was developed as a bottle-necked round to alleviate this "problem". I call it a "problem" because I've not heard of any such problem with modern components, and the 3-1/4" is still much more widely produced than the No 2. As Huvius said brass for the 3-1/4" is much more easy to find and can be loaded with reduced loads that can allow dialing back the power for almost any hunting, deer or hogs and larger. That's why I'm building a 3-1/4" for myself at the moment, components are more widely available and in many cases cheaper than pretty much any other vintage chambering for doubles.