Well, I would investigate 2 possible problems.
Case head separations are caused by there being too much of a gap between the case shoulder and the chamber shoulder. It is not defined as headspace since belted magnums headspace on the belt until the case has expanded enough so that you can then headspace on the shoulder. If you have a large gap on new cases then that would cause a lot of stretching at the web where the case body goes to being unsupported
When you fire the load, the firing pin slams the case forward, the case body expands and grips the walls, the pressure forces the case body back to the bolt face and the case thins at the pressure ring/web. A large gap at the shoulder is not uncommon and I have measured as much as .030". That is a problem with undersize brass as much as it is with oversize chamber.
When you Full Length Resize and push the shoulder back more than absolutely necessary then the thinning will happen every time you fire that case. As the brass becomes thinner there it will expand more readily and springback less. It would be good to know how much that clearance measures between the NEW case shoulder and the chamber shoulder.
The other thing is that in the few Remingtons I have reloaded for (7) the chambers have all been large enough so that it was a chore to size the fired case body in the die (normally Redding Body Dies) and there was a lot of scraping going on. This is not a problem in itself. Remingtons also tend to have a short shoulder and a long throat on an average IME but all are good shooters.
IMO, if you are having case head separation problems then you have a sizing problem.