I have a Lothar Walther Contour #722 on my 7x57 (not a Rigby) with a muzzle diameter at 25" length of 0.650" and I sometimes think it is a touch heavy for a stalking rifle.

http://www.lothar-walther.de/268.php

I wanted that length though as it gives a longer sight radius when I use the open sights. (bead front and ghost rear sight on the rear scope base)

On the other hand:

I have the Contour #718 in .243W on my sons rifle. Muzzle is 0.591" and even with a 6mm hole (which makes the barrel thicker than one with a 7mm hole) it is waifer thin. Too thin IMHO and better suited to a specialised mountain rifle. It heats up quickly after only three shots in the higher pressure .243W.

If I was to build again I would try for a rifle with a 25" barrel of around 0.600" - 0.620" like the shilen #3 contour.

http://www.shilen.com/contours.html Note the barrel weights too.