The premise that keeping the gap between the end of the case neck and the end of the neck part of the chamber to a minimum is an assumption that is not born out on the target. I recommend that you listen to the six segments that Jack Neary has on Youtube. In one of those segments he explains that even though his reamer is for a 1.525 max case length, he trims to 1.490, and based on his experience trims often for best results. Keep in mind that Jack is a truly world class competitor, and member of the Benchrest Hall of Fame. As for your "expert" that advises that you trim to chamber length, not only is he full of it, but he is giving potentially dangerous advice, given that some of the clueless out there are following die manufacturers bad advice to adjust FL dies to touch shell holders. That may result in cases that while of correct length, have their shoulders too far back For their particular rifle(for rimless cases) allowing the end of the neck to be crimped into the bullet as the case is driven forward in the chamber initially, during firing. On the other hand, the video work was quite nice.
