Happens to me after I have reloaded a case several times. Usually I just throw that case away.

Call the makers of your dies, they have techs to answer questions.

When all is said and done it can ONLY BE ONE OF TWO THINGS; either your sizing die is not reducing the neck diameter enough OR the expander ball is expanding the sized neck to much. If you neck turn your brass inside or out to much this will reduce neck tension. Firing a case and sizing multiple times makes the neck wall thickness Thicker Not thiner.
Remove the decapping rod size a case deburr the case mouth an measure the inside of the neck. .306 or smaller.
Measure the expander ball .306 .3065 is about right. All the rest is unimportant.
I have never anealed a piece of brass EVER. I have brass that has been loaded and trimmed so many times outside neck turning is required to avoid excessive bullet pull when chambered. The ONLY ammunition I crimp are revolver pistol cartridges (roll crimp) simi auto pistol (taper crimp) and shotshells. I don't do tubular magazine center fire or those would be crimped. Sounds like you need to review the basics.

cmcf, why have you never annealed? It seems to me that annealing helps maintain consistent neck tension. Not between rounds in a batch, they should be the same if loaded and fired the same number of times, but batch to batch. Is that not important to you? What about case life, not that cases are all that expensive unless you are a high volume shooter.

Short neck of 300 Winchester might be to blame. I have had magnum and 3006 rounds that recoil caused bullets to slip backward in their case (while in the magazine). I do not have a 300 Win but I do have the WSM 300. No problems so far with those reloads.

Edp;I have never seen the need for anealing. And from my research on the subject temp control is critical to get a consistent result. I have seen neck splitting in straight walled MAGNUM PISTOL cases that required a heavy roll crimp. I have never had a bottle neck rifle case split. BUT I only fl resize once, when brass is new unprimed. I have some. 270 win. That have been fired a dozen times and more. I neck size only and use the rifle the ammo is to be used in as a guage to set my sizing dies for a snug fit in the chamber. This minimizes the amount the brass is worked and the amount of stretching. As I posted before I have cases that I had to turn the outside of their necks because the wall thickness was such that it was TIGHT when a bullet was seated and the round was chambered. For what it's worth, The late Richard Mcquisten, a very dear friend, won the National Hunter Bench Rest Championship in 2008 with a rifle be built. We discussed the pros and cons of anealing rifle brass. His position was use good brass. Turn the neck when necessary, when it became necessary again throw it away.
With belted mag cartridges I consider five reloads to be maximum. This is due to incipient case head separation not brittleness. PS I have a .300 win.mag and have never had a problem with insufficient bullet pull, (neck tension).
I only load 180 gr. Sierra spitzer pro hunters flat base. They have been hammered by the front of the magazine box till the soft points were well flattened and no movement.

I tried neck sizing my 22 250 cases. When your in a pasture checking traps, spot a coyote running off, its time to pick up the rifle, rack the bolt and load it. It aint that easy. They fit tight. Don't just slide right in. Accuracy improves but not enough to be the deciding factor on whether or not a coyote gets pelted. Groups the size of a quarter reduced to size of a nickel. Not worth the effort for hunting IMO. brass life I've no doubt is better. Guess we all make choices. I'm going to full length size and crimp my 300 win mag. Shoots accurately, loads easily, no bullets seating deeper in the magazine.

P.S. never heard of annealing till I read it on TMan. Never been a problem not doing it and I aint gonna fix what aint broke.

anneal your brass, its work hardened.
RR

what happens if you don't rr?

The bullets unseat with new, unfired, once-fired, and more-than-once fired cases.

On both Norma and WW cases.

No, I'm not seating the bullets crooked or past the ogive. I use a Forster Ultra bullet seater die, and run-out is excellent.

Thanks for all the other comments. I've never annealed in the past, but I don't reload rifle cases more than three or four times.

Jim

Given that the problem exists with new, unfired brass, that seems to me to point to your expander, as you originally suspected. You can measure it and compare with my 30-06 RCBS expander measurement of .3065".

NEW UNFIRED! WORK HARDENED??
Them lil suckers must be working out in the box while it's still on the shelf!
HaaaaaHaaaa yea right.

James said, about 1 in four does it.

That part is hard to explain.

Sounds random, with all types of brass. If sizing the brass all "feels" the same and mic's good, put the mic to your bullets. I've never had a bad batch, but ya never know.

Thanks for all the suggestions. It's happened with more than one batch and brand of bullets (both Nosler and Swift).

The idea of comparing the expander ball measurement to the expander ball on a .30-06 is excellent. I happen to load for that round too, and have an '06 die set on the shelf. I'll have to mic both balls when I get the chance. Thanks for your own measurement, Ed.

Jim

It springs back, ya lose neck tension. Brass is made of copper and zinc, copper softens when annealed and gets hard and springy when worked, I anneal every 4-5 loadings, brass will last many loadings if ya take care of it and don't run high enough pressures to lose the pockets.
RR

perhaps a bad batch or improperly annealed from the factory, but its a brass problem, not anything in the loading process
RR

It's happened with three different batches of brass, two from Win and one from Norma.

It's got me befuddled. Doesn't happen with any other cartridge I load, from .22-250 to .416 Rem and .45-70. That's why I think it may be something to do with the die or expander ball.

Jim

I had a bad batch of 243 brass made by Winchester many years ago with this same problem. I started doing a slight crimp on everything I load since then, and I haven’t looked back. To me, crimping, is a good insurance policy.

Starting to sound like you might have a die set that is right on the edge of being out of spec. How old are the dies and how long have you been using them? Did you get the set new or used? New brass does or should have a tolerance spec. Try to get a standard micrometer and a tubing Micrometer to check and compare the neck wall thickness of your brass (new unfired or once fired) to get a handle on any inconsistencies. You could have an issue with a die that has a neck diameter that is large and some of the brass is on the small side. Out of spec dies are not common. I have only seen one set that I can remember, a set of 223 RCBS. The showed the same problem you are having.

glue