I am by no means an expert on recrowning barrels, but there are a few ways to do it. It's possible to recrown a barrel with sandpaper and ball bearings, or grit and round-headed brass screws. That having been said, I would rather spend around $100 at Brownells to order a chamfer cutter, brass pilot in the appropriate caliber, and drill bit adapter.
If you aren't sure what the rifling is, read AnatomyOfABullet before reading this article. Additional terms used here:
Crown: The crown is the 'business end' of a barrel. As the point where the bullet leaves the rifling, it is absolutely essential that the crown be completely even. In an even crown, the expanding gasses which propel the bullet are exerting equal pressure on every edge of the bullet. In an uneven crown, the gasses start to escape on one side a fraction of a second before it does elsewhere. The end result is much like what happens when you push on only one handle of a two-handled wheelbarrow: the excess force pushes the bullet to the side more than the other. The result is that instead of spinning evenly about its axis, the bullet starts tumbling rapidly. In radical cases, the bullet will tumble sideways immediately after exiting the barrel, resulting in what's called "keyholing". I own one Mosin-Nagant rifle with a crown so off-center that the bullets tumble 90 degrees within 10 yards, resulting in clean sideways silhouettes of bullets passing sideways through the target. While it would probably inflict grevious injuries at point-blank range, compared to a standard Mosin, it's still undesirable.
Causes of Crown Damage
One of the most popular causes of damaged crowns, is the practice of cleaning the barrel by inserting a cleaning rod down the muzzle end of the rifle, instead of cleaning from the chamber side. Scratching the crown repeatedly will tend to wear down one side or the other unevenly, resulting in gross inaccuracy. This is very frequently encountered on M1 Garands, but can happen to any rifle.
Barrel Crown Angles
Crowns are frequently cut at 90 degree (completely squared off) angles, 45 degree angles (common on pistol barrels), 11 degree angles (considered optimal for match shooting), and 5 degree angles (only commonly encountered on a few brands of revolver).
Typical Crowning Equipment Set
These tool sets are available from Brownells, Midway, etc.
Cutting bit: A cutting bit, which is more or less a drill bit with a hole in the center for a caliber-specific pilot bit and a threaded protrusion for a handle, does the actual cutting on the crown.
Pilot bit: Available in either steel or brass, this is a smooth cylinder bored out to the same size as the barrel. This serves as a guide to ensure that the cutting bit doesn't chatter from side to side in the hole. Brass is a softer metal than steel, but the rifling is likely to damage the pilot as it rotates in the shaft. That having been said, a steel pilot bit is far more likely to do further damage to the barrel's crown.
Handle or adapter: These are used with either a T-shaped handle (which looks kind of like it came out of a tap and die set), or an adapter to connect the cutting bit to a drill or drill press. Either way, it's necessary to rotate the cutting bit.
The PTG system uses a cutting bit which looks like a screw, but is used in conjunction with a can-like guide. The screw-like cutting bit is placed in the center of the barrel, and the barrel is turned under the cutting tool to obtain a consistent crown.
RW Hart lapping system
This is like the conventional crowning set, only without the cutting bit. With this, you apply lapping compound to a piece which resembles a pilot bit, to gently sand away imperfections.
-- SeanNewton - 23 Jan 2008