Gentlemens:
Backed out primers are caused by a number of factors working in concert. One thing for sure and for certain is if the primers have backed out a noticeable amount, you have excessive headspace.
With a rimmed case like the .577-450, the case rim fits into a recess that, in a perfect world, would be exactly the same depth as the rim thickness. In the real world the depth is always greater by a small amount and as long as the difference is small it is of no consequence. In fact it is necessary.
When the firing pin falls against the primer the whole case is pushed forward as far as it will go, which should be only a few thousandths with properly matched cases and chamber. When the priming compound explodes the primer cup is driven backwards against the breech while the case stays still. The primer now protrudes from the case by whatever amount the rim recess is greater than the rim thickness.
When the powder charge subsequently explodes (this all happens so fast it seems to be all at once, but is indeed in sequence) the pressure pushes the case back against the breech, reseating the primer. The case goes backwards and the bullet goes forward and all is right with the world. When you remove the case the primer is where is was when you put the cartridge in the chamber.
With a low pressure load it sometimes happens that, when the main charge explodes, the case expands and grips the sides of the chamber and the pressure does not rise high enough to either slide the case backwards or exceed the yield strength of the brass and stretch the case so the head is against the breech. Hence the case head stays forward where the firing pin pushed it, not getting pressed back over the primer. When the case is removed the primer is proud of the brass.
A number of things can compound this effect. One is excessive firing pin protrusion coupled with the heavy striker falls of these old rifles. The combination gets every bit of forward case movement available. A second player is a rough chamber surface, which allows the expanded brass a good grip and keeps the case from moving back.
90 grains of 2f is not a real light load and should be enough to push the case back. You might want to check the chamber finish. Since you have noticeable primer protrusion you have excessive headspace. In the case of a rimmed bottleneck case like this you can make a shim to fit around the front of the case rim, the shim being of a proper thickness to use up the extra headspace. When fired the case shoulder will be blown forward, filling the chamber. There after the case can be headspaced on the shoulder and you will have no further problems.
Best of luck,
Glenn
