Partial neck sizing - a problem?

[Hayudog]

For consistent neck tension, you should size no more neck length than seated bullet bearing communicates with. From here, you can reduce neck sizing length incrementally during load development, if desired. This length greater/smaller is the true tension adjustment (not seating interference amount).

All new/unturned necks have thicker brass nearest neck-shoulder junction. This is referred to as donut area, which changes over reload cycles. It's detrimental to seat bullet bearing into it, and just as bad to bring it into tension otherwise. Sizing of it does just that, regardless of seating depth.

Following this thread and want to get ^ this concept straight.
So for example, with a heavier long bullet that seats past the shoulder/neck junction, since the neck is only partial sized, the bullet will not be in contact with the case... would that be correct?

 

[30BR]

Following this thread and want to get ^ this concept straight.

He is talking about seating bullets, and total neck tension. Usually the donut rears its ugly head in cases that have been neck turned. Reason being that the neck wall has been made considerably thinner than the shoulder wall. Over time, as cases grow with firings, this thicker wall flows forward into the neck area. Complete neck sizing will push the extra thickness to the INSIDE on the neck. IF you are trying to seat the bullet deeper into the case than the base of the neck, this "donut" will physically stop the bullet from going deeper. You then need to remove it by reaming, or running the case over a mandrel to push it to the outside where it can be turned down to match the rest of the neck. But if you are seating the bullet ABOVE that contact area, it plays little role.
When he talks about not fully sizing the neck, he's leaving that donut area untouched, so that it won't interfere with deep seated bullets (depending on the thickness of the donut relative to the neck wall), as well as limiting the length of the neck that is sized to grip the bullet. If you can't make the ID of the neck larger, by not sizing as much of the neck, there will be less overall clamping force exerted on the bullet the less it is sized longitudinally.
Hope that helps..

 

[Mikecr]

I'm not thinking it's an over-pressure problem unless there's something I'm missing. Regarding the chamber, this is a custom build and the chamber was honed by the smith using a new, custom hone - I was thinking that perhaps the chamber was honed a little tighter than SAAMI, given the seating problems, rather than being sloppy. Is there another way to think about this?

Good information.
As far as "another way to think about this": excess upsizing leads to excess down sizing. They feed each other.
Where cases have expanded on firing there will be a web-line. That is the widest point of your cases. Can you measure this point with new and fired cases for comparison?
It may indicate chamber clearances as very tight, or very loose if so.

Finally, you mention oversizing - since I was focused on bumping the shoulder and didn't size the body all the way to the base, I was thinking I was undersizing the case, not oversizing. Were you saying that by not fully sizing the body, I was leaving the case itself oversized?

That's not what I was thinking. You wanted more sizing of necks, and shoulders, and now bodies. More more more..
There had not been a basis for more sizing until you mentioned that you can't get the bolt to close due to interference at the webs.

Case webs are very thick, and do not usually grow a lot, quickly, -unless:
1. High pressure loads (excess single use web growth can be a pressure problem)
2. Sloppy chamber, allowing so much growth to begin with. Tight chambers don't allow so much growth unless #1/#3
3. Insufficient breech support, allowing chamber end to expand excessively on firing. The brass goes where the chamber goes.

Now you could go to a special small base die to smash the webline enough to allow rechambering. But that's not a 'good' thing to do. Brass thickness would be rolling upward, thick toward thin, eventually leaving webs thinner, feeding on itself. That sizing will never put the brass where it used to be, and pockets will open at a higher rate, so you'll be replacing brass at a higher rate.
A small base die will not cure causes 1,2,3, and I doubt you wanted to get into band aiding your custom gun.

Once the problem is determined, that's when you should decide how to proceed, band aid or fix.

 

[Mikecr]

with a heavier long bullet that seats past the shoulder/neck junction, since the neck is only partial sized, the bullet will not be in contact with the case... would that be correct?

On firing, the necks grow to chamber neck and spring back a bit. It's often the case that you can drop bullets right through fired necks. That's in no way a requirement, but many see that when donut area has grown thick enough the bullets will stop on it. If they are, then you dang sure do not need to be sizing donut area inward even more!
If donut area is not bad, and partially sizing above it, then the donut area is not binding or adding to normal neck tension. This, even while seating through it.

Unless you've turned necks and minimally size to remove & mitigate changes in thickness at the donut area, then it's best not to seat bullet bearing through this, and NEVER add to the issue with sizing of it.

This is what you get with a normal seated bullet but FL sized neck,, picture why this could be a problem:

 

[Mikecr]

In this pic you can see a partially sized fired neck, leaving 1thou interference(after springback) for seating. The sizing stops on seated bullet bearing(does not exceed it). The length of which (THE neck tension adjustment) is set as established through load development. This sizing does not bring donut thickness into tension, nor cause any binding on bullet base-bearing junction. If seating testing were to have caused me to seat the bullets a full 1/4" deeper in this neck, it still would not affect the set neck tension, as grip on the bearing would remain exactly the same (from sized portion only). At least consider these things with your sizing plan.