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. The barrel tapers down about eight to ten inches from the muzzle, then flares back out slightly. This puts most of the weight near the breech so the rifle balances well and isn't muzzle heavy, and the flare at the muzzle gives a graceful look and a bit more sight plane.