"The rubber o-ring around the .303 rim makes the word "headspace" meaningless."

Until it is so excessive that you get a case head separation because your o-ring doesn't provide any support past the web area of the cases.

Why not just seat your bullets long and fire form that way? Better yet why not get a #2, 3 etc. bolt head (assuming you aren't using a No.1 Enfield)?

And there is a big difference between loading for the 303 Brit and loading for more modern cartridges. Yes I use the "fireform to chamber dimensions" type of headspacing on many old low pressure cartridges, but I wouldn't want to do that with some of the loads folks are running in 6mmBR rifles.