The following is quoted from George Shumway's book Recreating the American Longrifle copyrighted in 1970.
Tapered-and-flared barrels or swamped barrels are largest at the breech, and taper to a minimum diameter 6 to 10 inches from the muzzle, than flare out somewhat to the muzzle. Barrels of this type not only balance the best in a rifle but they also look the best. German flintlock rifles of the 18th century almost always had barrels of this type. The flare or the enlargement at the muzzle is aesthetically pleasing, it provides a higher base for the front sight than a straight tapered barrel would provide and it adds a little mass at the end of the barrel, which is an aid in shooting.
Another advantage of the tapered-and-flared barrel is that it provides for a subtle slendering of the stock from the breech to muzzle which adds to the beauty of the rifle. It also aids in making a waist region that is architecturally pleasing. It is a fact that almost all of the old rifles of the flintlock period had swamped barrels though on many of the late ones it was a minor feature.
For shooting at moving targets, say a running deer, swamped barrels offer better dynamic qualities. The same principle is applied to the smallbore 22's that are used in the Olympic events where moving targets are engaged: the barrel of these rifles is larger at the muzzle than some inches back. Top quality shotguns also usually have swamped barrels.
To try to simplify:
Picture a barrel that has substantial weight on the muzzle end, it swings smoothly. On the other hand a barrel of the same total weight but that is muzzle light is much more difficult to swiing smoothly.
I've had two rifles with swamped barrels. A 38 inch .50 that was too muzzle light to hold steady on the target. I sold that rifle but now have a 42 inch .45 caliber that seems about perfect. Should make a dandy off hand rifle. Just received it and haven't had a chance to shoot it yet. There are a lot of people that don't like swamped barrels as they prefer a heavier weight at the muzzle.
A swamped barrel also has different barrel harmonics, and may lead to the barrel more consistently returning to the same rest point. This is essentially what the Browning BOSS system does to a barrel. It is adjustable to tune the barrel harmonics to the particular load.
Yes, that is the basic failing of a muzzle break. The BOSS system, however, can be had without the muzzlebreak. It's not the muzzle break but the adjustable barrel weight that changes the barrel harmonics.
I thank ya'll, not that I could afford one, I enjoy browsing the custom gunmakers websites. There are works of art in most of them, and it seems that most of them refered to their barrels being "swamped". Now that I know what it is, I understand the principles of how it might help, and how it could cause more problems for the stock maker. Lordy, they sure do look pretty though!
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