I have some of that stuff in .30-06 I used to load some ammo for my brother. Despite being well under max it splits and cracks in random places half the time. Higginsons didn't seem to care. An unusual lack of customer service from them in this instance.
I've got a bunch of those cases in 7x57/8x57/30-06 and 6mm Remington.
I've never had any of the issues you write about.
Are you annealing the necks after four or five reloads?
The 7x57 cases are used exclusively in a Mexican small ring 98 type Mauser, which has a rather large chamber (not unusual for Mexican chambers) The rifle was manufactured in Europe for Mexico.
I only reload 20 cases at a time for this rifle as I don't shoot it much, but I like to take it out a couple of times each year as it has some nostalgic memories because it was a gift from someone I respected a lot, over 40 years ago.
I have reloaded those cases at least ten times with a "standard'' low pressure load of 49.0 grains of W760, over BR2 primers, under 140 grain #1905, Sierra SPBT bullets. 2800fps at 39,700CUP or 43,000psi. This is a great load. Easy on the rifle and brass. Accurate.
I annealed the cases again, after the last time I used them, this summer. I neck size only and bump back the shoulder a few thou right after annealing.




















































