I've noticed some pistols have barrels that have a constriction and then a flare on the outside near the muzzle. The popular models that come to mind are 3rd gen Smith and Wesson 9mms, and keltec autos, and I know there are more. What is the purpose of this shape?
http://http://media.midwayusa.com/productimages/880x660/Primary/494/494979.jpg