The load is new Remington brass, CCI large rifle primer, 55 grains of H414 and a Remington PSP bullet. This load is quite low in pressure judging from the primers. I am shooting it out of a Ruger M77 which has never exhibited symptoms of excessive headspace. The issue is the cases are being foreshortened in firing and the primers are backing out as much as 0.015. I have a Dillon headspace gauge, the unfired cases do not show excessive headspace that is they are slightly proud of the Dillon gauge. The fired cases are below flush in the Dillon gauge, some slightly, a few thousandths, a few are really below flush, 0.015 as I mentioned. I have no idea the mechanism that is pushing these cased deep into the chamber during firing. Possibly the fine grained H141 is falling down the flash hole and entering the empty space between the primer and primer pocket. On firing the pressure inside the primer pocket pushes the primer hard against the bolt face and the case forward into the chamber. I know it is not the blow of the firing pin. I struck several cases several times with the firing pin and they didn't shorten the case a bit according to the Dillon gauge. Primers only had no effect again, according to the gauge. What is going on here?

The primer explosion is holding the cases to the front of the chamber against the force of the light load.
You don't say what weight of bullet is being used. You might increase the load in one grain increments until the cases quit showing the raised primers.

If your Ruger is the old M77 with the plunger ejector both the ejector and the firing pin strike are helping push the round to the front of the chamber.

you don't have enough pressure to swell the cases out to the chamber and flow back-wards enough to re-seat the primers.

If you lighyly oil the case it will go
forward from the primer, come back at low
pressure, and fill out forward avoiding
the "stretch ring".

quote:

you don't have enough pressure to swell the cases out to the chamber and flow back-wards enough to re-seat the primers.

Yup ..... and,

quote:

If you lighyly oil the case it will go forward from the primer, come back at low pressure, and fill out forward avoiding the "stretch ring".

Yup, Great advice, as the pressure in your load is actually too low.

I just reread your first post and you never said the weight of your bullet. With a 125 gr Rem PSP bullet the 55 gr chage is well below the book minimum. With a 180 gr that is about maximum. What weight is your bullet?
When your firing pin strikes the primer it pushes the case farther into the chamber.
Once the primer ignites it pushers the primer out of the case a bit. As the powder ignites, the pressure of the burning powder pushes not only on the bullet but in all directions. Cases being thinner at the mouth expand quicker than at the base. This causes the mouth to seal in the chamber and prevent gas from leaking back around the case. As the pressure builds the case keeps expanding back along the walls of the chamber untill the only thing left to do is push te base of the case back against the bolt. If the pressure is enough at this point the case gets pushed back around the primer.
In your case, pardon the pun, your powder charge isn't creating enough pressure to push the case back around the primer.
This often happens with charges at a minimum charge as listed in the books. Mostly with guns with a chamber on the big side of the limits and cases sized to the minimum limits. These give headspace specs on the max side of the limit.
I as, other reccomend stepping the powder charge up.
Leo