Mike has mentioned a target crown, but lets step back for a second. Manufacturers have learned that a particular ramp angle at the muzzle will deliver best accuracy. It's a counterbore from the muzzle into the rifling, and even if they are even, some angles have proven more accurate than others.
Rifles and high power pistols sometimes have to be re-crowned. That's because the hot, high pressure gases that push the bullet down the barrel escape irregularly as the bullet leaves the barrel, and may erode the barrel further.
Another cause of erosion that requires recrowning, if not rebarreling, is the erosion caused by a cleaning rod inserted from the muzzle. This is a common problem with the M1 Garand, for example, and sometimes a 1/4" has to be cut off, if the barrel can be saved at all. In the worst cases you may see keyholing, but more likely a rifle that was shooting 1/5" groups will start shooting 3" group over time. Throat erosion is usually less of a problem.
Most high power or service rifles will show a rounded shape at the muzzle, with the ramp not really visible. A target crown is recessed in a heavy barrel rifle such as a target rifle, to prevent the crown from being nicked.
On the Garand and similar rifles, the "bullet test" is used. A bullet from the standard M2 service ammo is inserted in the muzzle, and the distance measured from the muzzle to the case. If it goes all the way in, it's shot. A reading of 1/8" or more means it usually good for another 1,000 rounds or so.
When the previous owner of the CZ 85 Combat I bought complained that he could not get the pisto lto group well, one of the things I did was to have the barrel recrowned - it's usually $25-35.