Fired case neck question

[GrayTech]

I recently fired 20 Winchester factory rounds to have some once fired cases from my own 270 win. I load cast sized to .278.
The fired cases I bought used previously had to be full sized as they wouldn't chamber in my rifle.
The cases fired in my rifle have a smaller neck diameter than the bought fired cases. .278 boolit fits snugly in the bought case necks before sizing, but not in those fired from my rifle.
Belled a case mouth heavily with .278 expander and it chambered no problem.

Why would my necks not expand to fill the chamber when fired? Primers are kinda flattened but not excessive, so there should have been enough pressure I'm assuming. Am I missing something here?

[scottfire1957]

Different chamber/neck dimensions in different rifles.

A more worn reamer will produce a smaller chamber/neck than a new reamer.

Different reamer manufacturers.

[country gent]

What is your neck dia in your chamber? It may be low pressure and brass sping back combined causing this. What is your loaded round neck dia dimension compared to chambers neck dimension. ( loaded round measurement) If bullet dia is .278 then loaded neck dimension should be around .298 which may be getting close to chambers neck dia

[GrayTech]

A heavily expanded neck still chambers no problem, so the neck didn't fill the chamber when fired. Going to try annealing them first and see what happens. Hopefully less spring-back.

[GrayTech]

From the recoil it certainly didn't feel like low pressure. And primers are substantially flattened.

[GrayTech]

What is your neck dia in your chamber? It may be low pressure and brass sping back combined causing this. What is your loaded round neck dia dimension compared to chambers neck dimension. ( loaded round measurement) If bullet dia is .278 then loaded neck dimension should be around .298 which may be getting close to chambers neck dia
The fired cases I bought came from a different gun, allowing .278 boolit snug fit. What I hadn't considered is that the chamber may have been larger at the neck. This is likely as the fired cases won't chamber in my rifle without sizing.
Will have to do a chamber cast to get accurate dimensions.

Maybe I just need to use the expander M die I have on my fired cases and ignore this, but my mind is inquiring.

Anyone know if RCBS makes custom size expander button in 278 for FL die.

[Yodogsandman]

The cases fired from full power, factory ammo in YOUR rifle did expand to YOUR chamber neck walls. All chambers will be a little different. The once fired cases you bought were fired in some other rifle with different specs.

Low pressure, reduced load, cast boolit loads may not fully expand case necks to your rifle chamber neck walls before releasing the boolit when fired. You'll recognize them by the blackened, sooty necks.

Measure the outside diameter of those full powered cases fired in YOUR rifle. For YOUR safety, do not exceed that measurement in your reloaded rounds. Remember that the case neck has to be able to release the boolit and after release shrinks back just a hair allowing the case to be extracted from the chamber. If your reloaded round's neck has to be forced into the chamber, it won't be able to release the boolit without excessive pressure or a KABOOM. Luckily, cast boolits are slipperier than jacketed and most times will release without a KABOOM.

Basically, advanced cast boolit reloaders will try to cheat a bit for maximum accuracy by sizing the case neck to fit the chamber neck walls within .0005". They will also outside neck turn the cases to allow for a boolit to fit exactly into the freebore area and also for consistent neck tension. This is the minimum for safety in their rifles. They have exacting measurement tools to be sure of that. For them, their ammo is safe.

[GrayTech]

The cases fired from full power, factory ammo in YOUR rifle did expand to YOUR chamber neck walls. All chambers will be a little different. The once fired cases you bought were fired in some other rifle with different specs.

Low pressure, reduced load, cast boolit loads may not fully expand case necks to your rifle chamber neck walls before releasing the boolit when fired. You'll recognize them by the blackened, sooty necks.

Measure the outside diameter of those full powered cases fired in YOUR rifle. For YOUR safety, do not exceed that measurement in your reloaded rounds. Remember that the case neck has to be able to release the boolit and after release shrinks back just a hair allowing the case to be extracted from the chamber. If your reloaded round's neck has to be forced into the chamber, it won't be able to release the boolit without excessive pressure or a KABOOM. Luckily, cast boolits are slipperier than jacketed and most times will release without a KABOOM.

Basically, advanced cast boolit reloaders will try to cheat a bit for maximum accuracy by sizing the case neck to fit the chamber neck walls within .0005". They will also outside neck turn the cases to allow for a boolit to fit exactly into the freebore area and also for consistent neck tension. This is the minimum for safety in their rifles. They have exacting measurement tools to be sure of that. For them, their ammo is safe.
Thanks for the feedback guys. What I'm not getting is that a heavily belled neck chambers with no resistance. Think I'm going to have to do a chamber cast to clarify once and for all.

[plainsman456]

I bet that the cases are larger at the shoulder junction than those fired in your rifle.

I have 1 that has a chamber on the large side and it swallows them with no problem.

[44man]

Seems there is a misconception about boolit release from the brass expanding before the boolit has left the brass. Does not happen that way. But pressure will increase if the neck has no clearance and holds the boolit tight. Merely more case tension then you want. The norm is .002" so just brass tension on the boolit is the control.
Pressure will expand the neck behind the boolit as it moves, not before. The neck does not "jump open" leaving the boolit hanging in space.
Loss of sustained pressure from a fast powder will not hold brass to the the chamber walls either. That might not even push neck brass to the walls, peak has long gone.
If I shot the 270 I would start with 3031 up to 4350 and not Unique. Move peak farther forward. Sustain push.

[Litl Red 3991]

Obviously, both batches of brass differ from each other. How much they stretch and how much they spring back is going to differ.

The new stuff is starting off with an OD the die gave it and it's response to the bullet that was pushed into it. And it started from a different diameter than you old brass.

Anneal both batches and then the only difference will be in the alloys in each batch, and of course the wall thickness. There are a lot of minor details that don't get mentioned in discussions like this. Ever wonder why serious benchresters don't usually shoot mixed brass.