Headspace change FS vs NS riddle.

[2fruits]

Hi All,

Very new to reloading, wanted to get into it and ole COVID gave me the time/opportunity..

Anyhows, hope everyone's good. Got a quick question:
I have a Sako 85 chambered in .243 and a bunch of sako brass. Bought myself a necksizing die and a full size die and got to work with the plane to fullsize 15 or so for hunting and necksize bunch for target. Once done, i also bought a Hornady modified case to work out seating depths, and a headspace comparator. Before i began loading and seating, i wanted to know the difference between my modified case, Neck Size and Full sized so I could calculate the correct adjustments when seating, but found the following (very consistent across several cases) measurements:

Modified: 1.622"
Fullsized: 1.625'
Necksized: 1.623

What i don't understand is, why if the fullsized cases' headspace greater than the fireformed and only necksized cases? I would have thought that when fireforming, it would have expanded to fit the chamber, therefore, would be at maximum. if so, why are the fullsized larger?

I have not attempted to chamber a round yet, rifle is in secure storage for a wee while.

Many thanks in advance!

[marksman]

how l would answer is that when the case is being FLSed the brass is being pushed forward because of the reduction in the case diameter as it is sized
if you measure just behind the shoulder junction you will see the neck sized case will be a larger diameter

l would recommend doing the same sort of sizing for hunting and target as you will more than likely get 2 different velocities/pressures from FLS and NS because of differing internal volume, if you are going to FLS maybe this will help to get consistancy
viewtopic.php?f=13&t=11560

another thing l would suggest is to send a couple of your fireformed cases to BRT and have Stewart make the modified case from one of your fireformed cases
l would give them a ring about it PH: 07 38084862
http://brtshooterssupply.com.au/index.html

[straightshooter]

Chances are that when you neck size you are somehow modifying the datum on the shoulder of the case that is used by your headspace evaluation device.
Try repositioning the neck die such that it only sizes 2/3rds of the neck.
If that's not the answer then chances are that the neck die dimension is somewhat too tight and is slightly collapsing the shoulder. An ideal "feel" when necksizing in a conventional die is that you can barely sense the expander ball resistance on withdrawing the case from the die.

[SCJ429]

As Marksman said, when you reduce the diameter of the case Walls by sizing it you draw the brass out and it lengthens.

Are you using dies that use an expander ball? I use bushing dies for my 243 but find I can make some excellent ammunition with Lee collet dies.

What is the lenght of a fired but unsized case measured at the shoulder datum line? Are we only talking about a couple of thou there? If so, adjust your FLS die to push the shoulder back two thou because you are not bumping the shoulder with the die.