What is free bore:
Free bore is just that, it is the area ahead of the bullet ogive (throat) in a rifle barrel to which there is no rifling, once the cartridge is fired the bullet travels freely in the barrel until the bullets ogive contacts the rifling, hence free bore. (See Detailed Image Below)
Why add more free bore:
The case for free boring is made this way, if you have a bullet seated deeply in a rifle cartridge case, the bullet is actually decreasing case capacity which in turn lowers the over all velocity of that imparticular bullet and cartridge case loading.
By adding more free bore, the bullet is then able to be seated out further thus increasing the case capacity which in turns allows you to add more powder to the case and thus an increase in velocity, usually quite a substantial increase in velocity at that.
How much can it help:
Usually increases are substantial to the point of standard calibers will be within reach of magnum velocities and magnum cartridges really get to actually earn the name magnum.
A 26" barreled 300 Winchester magnum firing a factory 180 grain bullet chronographed 2,950 feet per second, after free boring allowing that same bullet to be seated all the way out to where the base of the bullet is even with the bottom of the case neck and more powder added to equal same pressure level as factory levels, the round chronographed at 3,250 feet per second.
Important points to consider before free boring:
To gain the full potential of adding more free bore to your rifle there are several issues that you must consider before the process.
1. You must handload your own ammunition.
2. Do you want to limit bullet weights in this rifle for optimum accuracy.
3. Will the magazine box in this rifle have to be altered to handle the longer ammo.
4. Will the increase in velocity that you gain in this process substantiate all the above.
All custom Weatherby rifle cartridge chamberings should be free-bored, rifles manufactured by Weatherby for Weatherby cartridges are free-bored, as this is how they achieved higher velocities years ago. Clymer reamers carry a full line of throating reamers.
This job best left to a qualified gunsmith, riflesmith.