Was the correct headspace guage used? There are two types of 8x57 guages they differ on the shoulder angle. Not sure if it would give you a false safe headspace reading, though it could do the opposite.

I would look for a shiny ring around the head of the cartridge to indicate excessive headspace- if you see a shiny ring, bend a paperclip into an 'L' shape and sharpen the end of the short leg of the 'L'. use it to feel for a depression on the inside where the brass might have stretched near the casehead. If you don't feel anything on severl cases, I wouldn't worry about it.

It doesn't suprise me too much that a remington load for 8x57 would have a primer backing out. A 170 gr bullet shoul dbe pushed almost nearly 2800 fps in a normal load and remington pushes theirs around 2400 fps. Its severely underloaded and the primer needs a certain level of pressure to seal to the primer pocket. Do you notice any black soot around the primers? Its a dead giveaway that gases are leaking past the primer.