Forward of a rifles chamber(where the case ends) is a cylindrical section that is cut very slightly greater than the bullet diameter. This area is called freebore. Since bullets vary greatly in the lengths of their bearing surfaces, the freebore length can be adjusted to accomodate certain bullets. Freebore is usually ground into the chamber reamer and is fixed. Some gunsmiths adjust freebore in a seperate operation using a throating reamer. There is a lot of overlap, but imagine that you have a long freebore cut in your 300 winmag to accomodate the 240 gr sierra bullet. Then you try and shoot a 180. It will probably work, but you'll be forced to load way off the lands(short bullet, short neck of the 300wm). Conversely, if your rifle has a short throat, for 180's, and you want to run the 240, you'll have to seat it deep in the case. The winmag in particular has a rather short neck, making freebore specs a little more important. Sometimes there are magazine length requirements, where freebore would be adjusted so your range of usable seating depths with your chosen bullets produce a cartridge that will actually fit in your magazine. There are some other dimensions that come into play, but freebore is the meat an potatoes of it.
