I got it!, I looked at some photos of some greek structures around Washington DC, and Athens, and as stated, they are slightly larger at the waist or middle to give a correct vertical view as perfectly straight. I also got out my Hollis and it measures slightly larger at the muzzle then in the middle for I supposed the reasons kamilaroi, and Mike state....I then looked up contour numbers used by the barrel makers. I think the contour that I saw on the Holland & Holland was something between a #3 and #1 but not quite the #2. For such a large caliber (577), it was amazingly slender "looking". A bit of the Greek philosophy of entasis theories I suppose..Beautiful art!!