What exactly does recrowning do for a rifle barrel?

[longwinters]

I never cared about the gunsmithing thing but am getting more interested as I read things on this site.  What exactly does it do? Does it really help?  When would a rifle barrel need it done?  

long

[ringo]

In a perfect world, the bullet leaves the end of the barrel with all its body engaging the barrel until the instant it exits followed by hot high pressure gasses.  If in our real world  the crown becomes worn or damaged,  the bullet will engage the barrel a tad longer on one side rather than the other.  The barrel resistance is not constant and the bullet is "thrown" from its intended path pushed by those gasses, in extreme cases it actually tumbles, resulting in keyholes in your target.   This is why one cleans the barrel from the breach end, to minimize crown damage.  Recrowning can put an inaccurate barrel with crown damage back into service producing acceptable groups.  If you throw throat erosion into the mix, think in terms of a rechamber or a new rebarrel.

[gunnut69]

I've found that some barrels, notably button rifled barrels such a Remington uses will sometimes(not always) shoot better if recrowned if they have a bit of shooting under their belts so to speak.  I was at a loss for an explanation but was told by someone I respect that the button rifling process leaves a work hardened 'skin' of brittle steel in the bore.  This brittle material sustains mivro fractures from the concussion of firing and over time the fractures, appearing as chips around the bore, degrade the smoth release of the bullet and accuracy suffers.  I believe the chips allow high pressure combustion gases to leak as the bullet exits the muzzel which causes the bullet to be a bit more unstable.  They eventually restabalize but the dispersion introduced cannot be remedied.  A good crown is very important to the accuracy of a firearm..