I do agree with the last statement regarding full length sizing and rimless cases. Most FLS dies will size too far in most instances, if you set it up according to the instructions only. They have to size to minimum headspace dimensions to cater for all possible firearms that the brass could be used in, which means in many instances the brass is being resized too far and creating excessive headspace and poor case life wrt incipent head separations.

FLS dies need to be set up to suit the rifle and this is easy enough to do once you have some fired brass to test the die settings with - only resize as far so that chambering of the case results in slight resistance when closing the bolt (you know the shoulder is contacting the front of the chamber that way). Many people are aware of this and many aren't.

(Sidenote: There are also FLS dies available for using brass in semi-autos where some extra headspace is required to ensure reliable feeding. The dies will resize the headspace length below normal. Brass is generally only fired once and then discarded because it will likely have the tell tale ring above the web. Having said that, when I had my M14 [back in the good old days] I still set my FLS die up to suit the rifle and never had a malfunction and got good case life - about 15 reloads if I recall correctly.)

450 Ackley - what sort of life did you get out of your 303 cases? I've never seen better than 5 or 6 reloads where headspacing was done on the shoulder (only because it was excessive off the rim).