You have a chamber and do not know the length from the bolt face to the shoulder, you have a full length sizer die and do not know the length of the case from the case head to it's shoulder after sizing and or after firing, and you are being told to purchased a neck sizer die, the next thing you will be told to do is go back and full length size the case and start over, and still I ask "How do they do that, fire a case 6 times, THEN, full length size the 6 time fired case to start over".
For most keeping up with two thought at once is not easy, back to the a full length sizer die and "do not know the length of the case from the case head to it's shoulder after sizing and or after firing" because! the next bit of advise you will get is 'SPRING BACK', I call memory/recovery, by the time my cases are fired 6 times the memory has been knocked out (Hammered) of them and they resist being sized. It is possible for my fired cases to whip my press, shell holder and die, back to knowing the length of a sized case, if a case whips my die, press and shell holder I can measure the amount, in thousands, of the case that did not get pushed, shoved or crammed into the die, or put another way the amount of the case that does not get sized is between the bottom of the die and top of the shell holder, those that make this stuff up call it spring back, jump back, recovery or memory, I fire a case once, I get once fired cases, others get fire formed cases, I form first then fire, they fire the case 6 times and then start over, and still I ask "How do they do that, the case has been fired 6 times?"
Anyhow, I know the length of the chamber, I adjust the die to the shell holder and control the amount of sizing, because time is a factor and there is an ever so slight difference between sizing a case to the chamber and neck sizing and time is a factor etc., etc.. my bullet is just milliseconds behind the bullet when compared to neck sizing, but when keeping up with more than one thought at the same time, anyhow, I am a big fan of the versatile full length sizer.
Again, I purchased a 30/06 RCBS die set that included a neck sizer die, $15.00 dollars at the MARKET HALL GUN SHOW 2 weeks ago, useful? Bargain? Or nice to have? The problem is the die does not have case support and is not large enough in diameter for wildcats and too long for other 30 cal chambers.