Ks,
I don't think it is as simple as saying one metal is softer than the other and therefore the softer metal will be pushed around by the harder metal. Surely there are other factors and forces at play here.
My Grendel cases have been sized by a die without the expander ball. The neck has been sized from the outside only. For hard metal pushing around soft metal to be of concern the neck must not be concentric. Because if the neck is perfectly round then hard metal is pushing soft metal equally in a perfect circle and everything is fine. But let's say the neck was badly dinged on one side by the deflector on ejection and you went straight into seating the next bullet (unusual hypothetical I know). Seating might also be down to the elasticity of the neck vs. the incompressibility of a solid cylinder. The 'give' in the cupronickel jacket reinforced by solid lead tamping vs. the malleability of brass with no external tamping.
Regardless, when I first removed the ball I measured the outside of the neck with micrometer to see how concentric the neck ended up. Turning the case a few degrees at a time in a full circle and realised that it was concentric...You don't need to size the inside as well. This now allays any concern that the case is going to deform the bullet unevenly as it goes in. The results on the target which is the acid test also tell me nothing has degraded.
As for using the bullet as a 'mandrel', every time we seat a bullet it is by definition a mandrel. With, or without the expander ball a bullet going into a sized neck acts as a mandrel to expand it slightly. For without being a mandrel it will not be held by the neck.
Centrefire's method of using proper mandrels is the precise way of doing it but my 'poor man's version works for me. If it was for a bolt gun in Bench Rest competition then sure, all options are on the table.
I don't think it is as simple as saying one metal is softer than the other and therefore the softer metal will be pushed around by the harder metal. Surely there are other factors and forces at play here.
My Grendel cases have been sized by a die without the expander ball. The neck has been sized from the outside only. For hard metal pushing around soft metal to be of concern the neck must not be concentric. Because if the neck is perfectly round then hard metal is pushing soft metal equally in a perfect circle and everything is fine. But let's say the neck was badly dinged on one side by the deflector on ejection and you went straight into seating the next bullet (unusual hypothetical I know). Seating might also be down to the elasticity of the neck vs. the incompressibility of a solid cylinder. The 'give' in the cupronickel jacket reinforced by solid lead tamping vs. the malleability of brass with no external tamping.
Regardless, when I first removed the ball I measured the outside of the neck with micrometer to see how concentric the neck ended up. Turning the case a few degrees at a time in a full circle and realised that it was concentric...You don't need to size the inside as well. This now allays any concern that the case is going to deform the bullet unevenly as it goes in. The results on the target which is the acid test also tell me nothing has degraded.
As for using the bullet as a 'mandrel', every time we seat a bullet it is by definition a mandrel. With, or without the expander ball a bullet going into a sized neck acts as a mandrel to expand it slightly. For without being a mandrel it will not be held by the neck.
Centrefire's method of using proper mandrels is the precise way of doing it but my 'poor man's version works for me. If it was for a bolt gun in Bench Rest competition then sure, all options are on the table.