Trouble is the term 'headspace' is often used interchangeably, without an understanding of what it should be referring to. This can be
either the relationship between the locking lugs of the bolt to the bolt face, the distance between the bolt face and the shoulder/belt/rim recess (dimensions of the rifle)
or the external dimensions of the case.
View attachment 321999
The headspace of most rifles can be adjusted on either the bolt (locking lugs to bolt face) or barrel (changing the relationship between bolt face to case shoulder/rim/belt, either setting the barrel back. using a different thickness of recoil lug on a Rem 700 or adjusting the locking shoulder nut on a Savage style action).
With reloading, we (usually) concern ourselves with adjusting the case dimensions to fit the rifle. Several companies sell 'headspace gagues' which further add to the confusuion.
L.E Wilson makes solid case gagues which allow you to compare the external dimensions of your brass to SAAMI spec. If the brass is too large, it juts out. If it is smaller, fits into the gauge and a recess is visible.
Hornady on the other hand offer attachments that can be fitted to calipers to provide a
relative measurement. They do not provide a definite measurement on the actual, external dimensions of the fired brass or reflect the internal dimensions of your chamber. They are strictly for comparison only, and the measurements obtained on one set of attachments cannot be carried over to another set.
The most important (and only IMO) dimensions worth worrying about are the internal dimensions of your chamber. Taking figures down on a piece of paper does nothing for me, either the round chambers or it does not. We want the former.
With that out of the way...
Most 'regular' bottleneck rifle cartridges should headspace off the shoulder. (1st image)
A rimmed cartridge will headspace off the case rim. (2nd)
If your rifle is a factory rifle
@Jura , its probably had its chamber cut to headspace off the belt. (3rd)
Ignore the 4th.
So you want to make the belt redundant by firing the shoulder forward, creating the condition shown in the first image.
Keep in mind that both the 7mm RM and 300WM are decendants of the 300 Holland and Holland, which has comparably little shoulder angle and relies entirely on the belt to headspace.
The initial firing that you are worried about (which creates condition 1 after a belted case is fired with a stout enough load) which will be of little to no concern, since the shoulder will be blown forward, acting like a rimless bottle-neck case from here on.
Believe it or not, most factory ammo, whether belted or not, experiences the same thing, as the cases are made either close to or approaching minimum SAAMI/CIP dimensions so they will chamber in most rifles. So you will be always be stretching the brass by a certain amount on its first firing.
In some cases you will even get a false pressure sign via an ejector mark on the case head when firing virgin brass.
This is sometimes reffered to as 'bolt slap', whereby the bolt face has stopped the case head from going further back during the firing process, as the brass case creates a temporary seal with the chamber before sprining back. The firing pin has driven the case forward slightly towards the shoulder, which stopped forward movement, the anvil has contacted the primer compound and set off the process of ignition
Whilst some would like to think their case sits perfectly parallel to their bore axis when the cartridge is chambered, this is simply not true, especially on a production rifle which is made to shoot factory ammo. There must be a degree of clearance between the chamber and case, if not the rounds would not chamber. Exception would be a tight 'match or benchrest' spec chamber where case necks are turned down to order to generate a tight, verging on interference fit.
There was/is also a school of thought that only neck sizing fired brass, to the point that you required a crush fit of the brass to chamber it, was somehow beneficial to accuracy due to a tighter fit in the chamber, and presumably longer brass life due to the lack of sizing (cold working).
This was shown to offer little discernable benefit in accuracy, but place a much greater strain on the firearm, as the locking lugs would usually gall up when camming the bolt closed and crushing the brass against the shoulder and chamber walls.
Back to topic, the point of the Larry Wills/Innovative Tenchnologies die is to squeeze down the small portion of brass, just ahead of the rim, which is now left unsupported due to the shoulder becoming the headspace datum line. This unsupported area is likely to expand, as it now sits outside the chamber. The die helps as it allows you to chamber rounds which would have otherwise become 'oversize' when firing in this condition.
The area ahead of the belt is much thicker when compared to the portion between the upper case body and shoulder, and will in theory be more resistant to repeated cycles of firing and sizing. However, unlike a shoulder, this portion of the case should not be annealed as it will greatly compromise the integrity of the case.
All in all, I'd not put extended brass life down as the be all and end all of reloading. It is a consumable after all!
I'd strongly reccomend tracking down 'Principles and Practice of Loading Ammunition' by Earl Naramore, which
@Muir has reccomended previously. All of above issues are easy to understand once the whole process in explained from start to finish.
Hopefully that was easy enough to follow (unlike the Wikipedia copy-paste drivel that
@Sharpie likes/liked to post...)
www.handloadermagazine.com
