603,
I don't have an issue with Redding either, and use their dies for other calibres. It does however seem that my Redding die, purchased some years ago, works brass differently to yours. Your squeeze is less severe than my Redding, and your expander ball leaves it with a .003" grip. That all sounds perfect, which makes me think it is the barrel.
I use a Redding bushing die for a 6.5x47L bolt gun and it does a good job. I use the .288 insert which gives .002 grip.
My thought about your necks being unsupported and the reason for the splitting still stands. Because if the necks have expanded hard up against the wall of the chamber (like they are supposed to) then the pressure takes the next easiest path and that is pushing the bullet down the bore (as it is supposed to). If pushing the bullet is too much resistance at the time then it will go the next easiest path and break off a bolt lug. If the pressure is being shared at the neck between the brass hard up against the thick wall of the barrel (as it is supposed to), and the thick steel of the barrel, then there's no reason for the brass to split - I mean, where does the gas go? Or in other words, if the brass has split when hard up against the chamber wall then it would be because the barrel has split too, and that of course is a catastrophic breach like a grenade.
You wrote that spent cases measure up to .300 which is right at the max of SAAMI specs for the Grendel. If we add a little more for brass springback, that means the chamber is actually larger than spec. The whole idea of using brass is that it is elastic. It expands to seal, but then contracts so it can be extracted. Spent cases have fireformed to the chamber but then contract slightly, meaning when fired they were maybe .001 more against the wall of the chamber.
Another thought is that if your chamber neck has been reamed too short then on loading the case mouth gets crushed into the bullet, preventing bullet release. The rest of the neck is unsupported and in that case it splits. The mouth getting crushed and gripping the bullet more often happens with hand-loaders letting their cases creep forward without trimming. The Grendel is very forgiving for case creep however and I rarely, if ever need to trim cases. I am thinking this scenario might be because of an out-of-spec chamber then; either too wide or too short. And because even if the load is hot it shouldn't split necks - more likely chip a bolt lug.
I am leaning to the necks being unsupported at the time of firing. If new brass is splitting at the neck on first firing then what sizing die you have is not the issue, and won't fix it. It's not headspace either because that splits the case down the body at the web.
I don't have an issue with Redding either, and use their dies for other calibres. It does however seem that my Redding die, purchased some years ago, works brass differently to yours. Your squeeze is less severe than my Redding, and your expander ball leaves it with a .003" grip. That all sounds perfect, which makes me think it is the barrel.
I use a Redding bushing die for a 6.5x47L bolt gun and it does a good job. I use the .288 insert which gives .002 grip.
My thought about your necks being unsupported and the reason for the splitting still stands. Because if the necks have expanded hard up against the wall of the chamber (like they are supposed to) then the pressure takes the next easiest path and that is pushing the bullet down the bore (as it is supposed to). If pushing the bullet is too much resistance at the time then it will go the next easiest path and break off a bolt lug. If the pressure is being shared at the neck between the brass hard up against the thick wall of the barrel (as it is supposed to), and the thick steel of the barrel, then there's no reason for the brass to split - I mean, where does the gas go? Or in other words, if the brass has split when hard up against the chamber wall then it would be because the barrel has split too, and that of course is a catastrophic breach like a grenade.
You wrote that spent cases measure up to .300 which is right at the max of SAAMI specs for the Grendel. If we add a little more for brass springback, that means the chamber is actually larger than spec. The whole idea of using brass is that it is elastic. It expands to seal, but then contracts so it can be extracted. Spent cases have fireformed to the chamber but then contract slightly, meaning when fired they were maybe .001 more against the wall of the chamber.
Another thought is that if your chamber neck has been reamed too short then on loading the case mouth gets crushed into the bullet, preventing bullet release. The rest of the neck is unsupported and in that case it splits. The mouth getting crushed and gripping the bullet more often happens with hand-loaders letting their cases creep forward without trimming. The Grendel is very forgiving for case creep however and I rarely, if ever need to trim cases. I am thinking this scenario might be because of an out-of-spec chamber then; either too wide or too short. And because even if the load is hot it shouldn't split necks - more likely chip a bolt lug.
I am leaning to the necks being unsupported at the time of firing. If new brass is splitting at the neck on first firing then what sizing die you have is not the issue, and won't fix it. It's not headspace either because that splits the case down the body at the web.