Originally Posted by
Ballistics in Scotland
You didn't say how far they were backing out, which could be important. It happens to the extent of the headspace excess of chamber (to the shoulder) over the same measurement in the cartridge. If it was excessive it could be caused by an over-deep chamber (perhaps a little dangerous when you switch to full power) or stretching or cracking of the action (worrying at any time.)
Now, however, those pictures look like the headspace is within safe limits.
I think this is primarily an effect of gas pressure, rather than primer pressure within its pocket. Try popping a primed but unloaded case and you might find the primer set back, but more often you won't. It doesn't matter very much though, as the things that happen happen a tiny fraction of a second later.
It happens because the adhesion of case to chamber, caused by the gas pressure, is too great to be overcome by the light pressure of this round. It actually indicates a very safe condition. With hot loads in the same rifle, a few things would happen which aren't very dangerous when it is on this scale, but might limit brass life. The head is forced back against the bolt face, stretching the brass. With a very tapered case that stretching occurs in the last fraction of an inch, but with the very straight-bodied Improved types stretch most of the way back to the shoulder.
I never did believe that cartridges like the .300H&H and .303 necessarily produce less accuracy or shorter brass life, and people used to get extremely high standards in both from rounds like the .30-40. But they need more accurate headspacing to do it.
Another effect of the primer being extruded and subsequently the case stretched backwards, is that it tends to mushroom the primers, leaving them with sharper edges around the edge of their pockets. That can falsify a sign of higher pressure than you in fact have. But it would produce an error in the direction of safety.