OzFclass

so named as the space between the front of the case and the 45 degree transition between chamber neck and freebore.
most reamers tend to be fairly generous in this dimension.
we like a bit of gap there to avoid a long case being crimped into the bullet on chambering a round, thus causing high pressures. we should trim cases to avoid this happening, and this situation can be measured with the appropriate tool.
I used to think that she shorter no man's land the better, due to bullet support being maximized.
the thought has arisen that too small a gap here could be a problem if powder fouling builds up during a string. maybe 0.010 minimum.
what think ye?
keep safe,
bruce.

Up to a point, I think it is more important that all cases are trimmed consistently, than the absolute length they are trimmed to. In fact one BR shooter in the USA mentioned that he trims his cases every reload. And this is a person who does a lot of testing in a tunnel, so does know what steps make a difference to accuracy, and what is redundant and can be dispensed with from the loading process.

Jack Neary touches on this issue too in his accuracy loading / shooting lecture (available on youtube, though I don't recall which of the six parts this is covered in. Its all great, so just watch the whole hour long series of 6 parts, part 1 here: http://www.youtube.com/watch?v=7SZWvn68bRU)

There was a trend in BR towards reamers with shorter freebore - supposedly to limit the carbon build up that causes the carbon ring at the end of the freebore/immediately ahead of the rifling. However in conversation with a number of good US BR shooters and gunsmiths, this theory has never panned out. Most good BR competitors I know ensure they maintain at least 10 thou clearance between the end of their case and the freebore of their chamber, and preferably more. Cases can be too long, but within reason the case can not be too short.

The only proviso I would add to this is that the vast majority of BR shooters are using a 6PPC, with the bullet "jammed" in the rifling to a greater or lesser extent. If the cartridge in question is set to "jump" bullets, then the support of the neck in the chamber may be a greater consideration.