I'm with the classical idea of beauty: all in the proportions. This is true of anything we look at--a building, a woman, a rifle, you name it. Long ago, the Greeks realized that there are measurable (and amazingly constant!) ratios that make something more or less pleasant to the senses: 1:1--symmetry, and the unison in music; 2:1--nice if a little blocky, and the octave interval in music; 3:2--perfect and interesting, and the fifth interval in music; 5:4--sensual and beautiful, and the third interval in music, etc...
Look at the ideal proportions of the human body, and these ratios appear. Ditto for the Fibonacci series (0 1, 1, 2, 3, 5, 8, 13, 21, 34....) whose curve follow the "universal" rule of harmony.
Classic rifles are beautiful because, for the most part, they follow these proportions. Look at a classic British double--the barrels (3/3rds) will have 2/3rds of length protruding past the forend, while the distance buttpad to tip of the forend will roughly equal the length of the barrels (1:1 ratio).
This is why, while undoubtedly nice touches, engraving and wood-grain take the back-seat to proportions in what make a rifle beautiful. A nicely engraved rifle with its elements out of proportion would look like a well-dressed woman with shoulders too wide or too short a neck. While a plainly-finished rifle with perfect proportions will look like... well, you can complete your own analogy ;-)
