free bore?

[sharpshooter]

Freebore can be a fickle *****.....zero freebore works well on some calibers, but disastrous in others. Weatherby used long freebore to reduce pressure spikes in his overbore magnums. The length of freebore is not as important as the diameter of freebore. I know a barrel maker that intentionally put 16" of freebore on a 6 PPC barrel. Much to our surprise, it shot just as well as one with .060" freebore.
To get a good idea of what your freebore length is, lightly seat a flat base bullet upside down and then chamber it to push the bullet in until it stops. The length between the neck and the end of the bullet is the freebore length.

[Deadshot2]

The length of freebore is not as important as the diameter of freebore.

All of my rifles have custom barrels with the freebore diameter so close to bullet diameter it's difficult to chamber a cartridge if there is any foreign material in the freebore area like a thread from a patch. The reamer is ground so the freebore is only .0002 greater than the standard bullet diameter for that caliber.

This allows me to seat bullets long enough to keep the boat-tail out of the powder space yet still have some jump. So far, every rifle I've had chambered that way shoots sub .25 MOA with a variety of bullets (as long as I get the load worked up correctly).