If your expander ball is the proper size for the cartridge you are loading, how can you have an undersized neck? Is there a high likelyhood you are overworking the brass with off the shelf dies?, absolutely, but an undersized neck?, not so much.
All's I meant was that if the bullet is too big or the expander too small, during seating the case will be stretched beyond the elastic limit of the brass and deformation will occur. And this deformation is highly likely to be asymmetrical if the bullet is free to wobble, particularly as it just begins to seat. (This is where the competition seating die will help enormously). Not that expander balls cause this. I was just covering the bases. Whereas, if the neck is expanded enough for the bullet, it undergoes only elastic stretching, rather than deformation, provided the bullet starts out straight.
I am under the naive impression that as long as your neck is concentric, the neck is expanded enough for the bullet, and the bullet starts out straight (this is where the M die flare comes in), the bullet will seat itself darn near dead straight, automatically. This might be where I am going wrong.
As you noted, making case necks concentric can be challenging. The act of pulling the expander ball through the case neck can cause stretching. And this is exacerbated by a sizing die that makes the neck too small to begin with. This is yet another benefit of the M die. There are too many to even list!
Imagine, if you will, a chinese finger trap. Your finger represents the expander ball. The harder you pull, the more the trap tightens. This is what the neck of the case CAN do to the expander ball. It should never be that bad, but you never know. A combination of thick brass and a slightly tight sizing die, and this phenomenon goes off the charts. With one of my dies, it was harder for me to pull the expander ball out of the cases than it was to size, even if I applied lube directly to the inside of the necks! Much harder; like a different order of magnitude. I checked the ball; it was the right size and smooth as anything. This is a lot of force and it makes the case neck stretch and expand unevenly. And I don't care when the expanding starts, like that Forster die. Regardless, I imagine the neck can be stretched unevenly, making the brass of the neck not all the same thickness and level of work hardened! So even if it's concentric, it may not be inclined to seat a bullet as straight.
But take the same cases, sized with the same die, but with the expander ball removed. The necks are just as small. Push an M die expander of the exact same diameter into the neck, and the chinese fingertrap situation is reversed. Even with no lube, the same cases expand effortlessly. It's an amazing difference to witness. The fact that the expander is a solid rod that spans the entire neck shouldn't hurt, either. I imagine that the cases expanded with the M die could be more concentric, on average. But maybe not. I have an active imagination.
I believe that bushing dies and collet dies can make necks even more concentric and even less worked. And as you pointed out, earlier, the pinnacle would be a custom FL sizing die and neck turned brass! But I think running any of these superbly straight cases thru an M die, after, just to give them a little flare, might possibly help, too. For the reasons I stated in my last post. Even if you already use a competition seater die.
If I were going to get serious about precision, I would go with custom FL dies to near exactly fit my rifle's chamber. Neck turned brass. The FL die would just barely undersize the necks. Then I'd run a custom M die through. It would be very gradually tapered to the right size, being much longer than a regular M die, to help it remain concentric. It would just barely expand the necks which are already near perfect. This would hopefully remove some of the variation in neck tension that would arise from trying to size the necks, perfectly, with the sizing die. And, of course, it would put that little flare on the case mouth. I'd use a custom made competition seating die shaped exactly to my brass and bullet. And finally, I would have to find someone who can shoot way better than me to see if this makes any difference!
