As posted over on Practical Riflery, a chamber being 'sub-minimum' diameter wouldn't cause a split or separation as you describe. Excessive headspace would, or over-pressure would. I am glad you have the issue fixed (you've fired the rifle since its return and it is, indeed, fixed, right?). Still, I think there were two separate issues at work here. Hopefully, correcting the one will have the impact of 'correcting' the other.