I did that with 7mm Rem Mag....here's the deal....
. USING NO POWDER or PRIMER, build a dummy round. Use NO powder until ur
Satisfied you have a winner and are prepared to load live rounds...

. First firing fire forms the case to yr chamber
. Adjust ur neck sizing die to size as little of the neck as possible...just enough to hold
The bullet AFTER you trim the case. In my rifle, would size about 1/2way down the neck
then trim back to a a few thousandths from max.
. Adjust ur seating die carefully...when you build your load, u want the oal as long as
possible. Back the seating die so the oal is over long. Use a match or lighter and blacken
Your bullet. Run it thru the action...do not force the bolt closed if iit won't go ! That means the oal is too long. What ur looking to do, is to look for the marks the barrel makes
On ur bullet when you chamber it...if u see marks, re-seat the bullet a couple thousandths deeper in the case and re blacken it. Repeat this procedure till you see no marks on the bullet after you chamber it.

What you've done at this point is to fire form tour case to chamber dimensions,
Seated the bullet deep enough in the case neck to be held in place but not to adversely
Affect it's performance with too much 'hold' from the case neck...and your cartridge is
Just short of the overall max length to shorten the jump from case to throat to rifling.

Use this dummy to set ur dies and lock em down for consistency. Next, load some live
Rounds. Be very wary for signs of high pressure when you fire ur reloads...start with lo powder charges ...remember....neck sizing takes a lot of the 'slop' out of ur ammo. Consequently, by pushing the max dimensions on ur ammo, it is natural that chamber
Pressures WILL rise. So start at minimum charges and work ur way up to where you get max accuracy with no signs of hi pressure.

I get 3/4 groups (5 shot) using this method in my Remington in .338wm...

You may get 2 or 3 reloads like this before you will need to full-length resize...you'll know it's time when the fit gets tight when u chamber a round.