Interesting, While I've not had the split neck problem I did have case head seperation after 3 to 4 firings. I'll give you my thoughts & results as maybe a answer to your problem.
When firing brass for the first time I would I would run them through the resize die to true them up and then trim to length, load and shoot for groups, (woking up a load). On subsequent resizing I would partial resize by sight so as not to set the shoulder back to far. The problem still existed, 3-4 firings and throw away. I purchaced a stoney point, now Hornady, headspace gauge. A good investment as I found I was, by sight, setting the shoulder to far back, with the gauge I was now able to set the shoulder back precisley .002 everytime, problem solved? No way, still was happening.
Thinking as rockislandrod, I thought it was just the nature of the beast and let it go as that. Of course that wasn't good enough, because I have reloaded and shot similiar cartridges, .300WM, etc, and never a problem. But, they were in different guns, so I took a new unfired case and measured it from case head to datum line, then measured a fired case. Difference was .020, fired case being longer and initialey stretching 20 thou. of an inch on the 1st firing, which caused the initial stretching, that cannot be reversed. (Please don't insert here about headspacing on the belt, doesn't happen that way.)
Now if you are setting up your die as the mfg. tell you and do have an oversize chamber I can see where you might be getting your splits in the neck, before, a head seperation. Now a solution and you won't know until you try: 1st find out "if" you have a generous chamber as I've explained in the text. If you do, on initial loading, that is new unfired brass, do your prep work, won't need to trim yet, Then neck your brass up to the next caliber, .338 or better yet .375 and then back down to 8mm, there by creating a false shoulder to headspace on. Firing your loads this way will fireform your brass to your chamber without any stretching in the critical areas. Then resize as normal, being careful not to set the shoulder back more tha .002 and trim. It worked for me as I was able to, up until now, go on my 6th firing without anything bad happening. Yes I know it's a pain in the a$$, But it's really not a thousand round a weekend varmint ctg. anyway.
One other solution, again assuming you do have a generous chamber is to send it back to Remington and tell them to set the barrel back, also don't take any of there bullsh!t that it is within SAAMI specs, send the proof. If you have problems with them, ask them if they need the press that one of their rifles let go due to sloppy tolerences.
OR send it to a 'smith and they will take care of it. Myself, I just did the brass resizing as you only have to do it once anyway.
This has been a long reply to simply checking your chamber dim. just might be it.
CRASH87
