^ I believe the principle is known as "entasis". Commonly known as the "rule of thirds" it isn't but an approximate ratio between respective elements of a structure.

Barrel "swamping" actually provides an effective means of steadying the aim by use of placing mass toward the muzzle (if used effectively). On that note I handled an H&H 360BP double circa 1875 (back action lock) of "Underlever's" this week that was truly the duck's guts.