Quote Originally Posted by Speedrat1 View Post
Chamber protrusion.... that amount of the chamber that is not contained in the barrel. IIRC the headspace measurement on a rimmed mag is approx. .221 and the figure you were given was .135 so...... .221 - .135 = .086 which is the amount of the cartridge that will be outside the chamber to allow for the extractor to get into the extractor groove.

hope that helps!
The belt is contained. The .135 is what a go gage will protrude from the barrel, which is about what the rim/extractor groove are plus a little bit(and some extra) for clearance between the bolt head/barrel. Factory bolt heads usually have ~.110 recess from the front to the face but they vary. With pre-fits they have to consider loose factory tolerances and add a bit of protrusion to make sure they will fit all rifles without bolt head/barrel interference issues since they aren't measuring the actual bolt face being used and chambering accordingly.