Freebore

Is freebore the amount of space in your chamber that exceedes the maximum oacl. For example the nosler handloading manual says that in 300 win mag my oacl should not exceed 3.340. But after several measurement of my chamber i come up with a measurement of 3.580 to the lans of the barrel. Does that mean that I have .240 of free bore and that I should reload my first set of three rounds at 3.570 and see what happens or is that unsafe because I am exceeding the maximum oacl that is stated in the manual?

Freebore is the space in the bore that has no lans and groves but is the same diameter as the overall bore. Roy Weatherby was one of the first to use a longer-than-usual freebore in his Weatherby chambered rifles, and the company still does it. There are two advantages of freebore. One is this helps to keep chamber pressures down since the bullet "jumps" into the rifling without resistance from the bore immediately. Another advantage is it helps increase overall velocity. There is a weak argument that a long freebore effects accuracy since the bullet is not as stable as it could be in a bore with much less freebore. That is really weak because Weatherby rifles, for instance, are very accurate with their longer-than-normal freebore.

You can attempt to seat your bullets not as deep as the COL states, and it still may chamber OK. The problem with that is you'll likely make a cartridge length that is too long to go in the magazine.