Let's measure up what we have. What counts for sure is the real headspace thats in our guns. To start I have measured two rifles, one a 270 Win and another a 264 WM.

What I did was to drop an unfired case into the next largest sizing die. So for the 270 Win I used a 30-06 seating die and removed the seater stem. The first rifle tested is a like new Ruger 77 in 270 Win. The new unprimed brass is WW and an average of three of each was taken. The fired rounds were also WW and have been fired a number of times and only previously neck sized.

The belted case is a 264 WM and the rifle is a pre-64 with a new Winchester factory barrel.

The fired 270 Win brass measured .002" longer by this system and the 264 WM brass measured .009" longer!

Post up your fired and unfired cases and lets see what's out there.

There are a whole bunch of explanations for your findings.
First, the 270 is designed to headspace on the shoulder and rifles and dies are manufactured with this as the determininf factor for headspace.
In the case of the belted magnums it is not unusual for the shoulders to move ahead by ten thou or so. Also the loading dies are made with some clearance at the belt to ensure that the belt is not reformed by the die. So, while the die can tell you how much the case shoulder moved ahead on firng, it doesn't necessarily tell you much about headspaceing which was managed by the belt recess in the chamber and it's fit in relation to the belt on the case.
Having said this I also must say that there is frequently significant headspace existent in rifles chambered for belted cartridges. The same applies to rimmed cartridges.
In the case of belted rounds the standard minimum measurement for the belt recess is .220" ahead of the breech face. I don't think I have ever seen a belted case which measured more than .216" from the head to the front of the belt. That gives you .004" headspace to start with. Now, if the rifle measures .222" (still within specs) and you have a piece of brass which measures .213" (not uncommon)then you will have .009" headspace. This is kind of a worst case scenario but is certainly not unheard of.
With a chamber for a rimmed case which is cut to the .063" minmum you may also have around .006" of headspace with the .057" thick rims which are common. My match 303 was cut to .060" and the rims all faced to .058 ( those that were thick enough to cut). Subsequent to the first firing, the cartridges are headspaced on the shoulder and by the generous 303 case taper!
In the end, your findings on the difference in headspace between your 270 and your 264 falls into the category of interesting but useless facts. Let us not forget the differences in individual rifles as well. I've seen a new 270 which featured .014" of heaspace on new Winchester ammunition so all is not perfect in the world of rimless headspacing either!
One friend chambers belted cases to heaspace on the shoulder. He uses a reamer with the belt potion removed then cuts the belt recess with a boring bar after headspacing on the shoulder of the brass to be used in the rifle. A workable system providing you don't buy new brass with a different shoulder measurement. I personally don't see the point. After fire forming everything is fine anyway and proper reloading technique will ensure it stays that way. Regards, Bill.

The bottom line is that with the design paramaters of rimless vrs belted cartridges and chambers that the brass will stretch less with the rimless. Of course the reality of actual chambers and actual cases exists but the inferior design of the belted case, when cartridges with adequate shoulders are present, is forever.



Many rimmed cartridges have the problem as well. It's a valid point but for the most part they are obsolete or used as such low pressure as not to be noticed.



It's true that some belted chambers are, by chance, pretty good. John Ricks brought this up and offers to make the chambers to fit the brass.



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Here are the measurements of headspace expansion for a 260 Rem in a Kimber 84M. This rifle will almost take the no go gage but not quite.

The headspace increased .004" on fired cases from new Remington brass. All fired primers were removed and careful measurements were taken.