Why do longrifle barrels have a swamped shape instead of being a constant diameter?
A swamped barrel is largest and strongest at the breech, where it needs to resist the force of the powder explosion, and gets thinner toward the muzzle where less strength is needed as pressure drops. This also keeps most of the weight near the breech so the rifle is not muzzle heavy, and the slight flare at the muzzle gives a graceful look and a bit more sight plane.