Expander die?

[Tex S]

When must expander dies be used?

Only for straight wall cases when using cast bullets?

On all straight wall cases, regardless of bullet type?

Only for cast lead bullets?

Should I expand 45-70 cartridges if using jacketed bullets?

Should I use an expander if loading 270 win while using cast bullets?

I know that's a lotta questions, but I'm just trying to figure out when expanding is necessary and when it isn't.

[LE-28]

You must use an expander die every time the resizing die is used, on all calibers and types of cases. The resizing die will resize the neck to actually smaller then needed and the expander die (flaring die) will bring the mouth of the case back out to with in .002-.003" of the bullet diameter. This insures proper neck tension on the bullet.

The expander die will also flare the top of the case to allow the bullet to start with out buckling the case.

I suppose their could be times where you wouldn't need to expand the case mouth, where just chamfering the inside of the mouth would be suffice, but I haven't seen it yet.

[Tex S]

You must use an expander die every time the resizing die is used, on all calibers and types of cases.

With the exception of bottleneck rifle when using jacketed bullets?

[jepp2]

I will assume your question is about dies like the Lyman M die or the Redding equivalent. And you are not asking about the expander on the decapping stem on a bottleneck cartridge full sizing die. If that is the case, my experience is different from:

You must use an expander die every time the resizing die is used, on all calibers and types of cases. The resizing die will resize the neck to actually smaller then needed and the expander die (flaring die) will bring the mouth of the case back out to with in .002-.003" of the bullet diameter. This insures proper neck tension on the bullet.

For bottleneck cartridges (mostly rifle) I don't use an expander when using a Lee collet die or a Redding bushing die with the right bushing and loading jacketed bullets.

I also don't use an expander when loading 357 Sig.

When loading straight wall handgun cartridges and using jacketed bullets, I don't use and expander die, and I only put 0.001" flare on the case.

I use a Lyman M type expander die for all lead cast bullet loading.

[Sure Shot Mc Gee]

Seating cast. There are other ways to bell the case mouth of a 270. Or any other caliber for that matter.

[Bart B.]

Expanders are needed anytime a case's mouth diameter is too small for bullets to properly seat into. It can be done in a sizing die whose neck makes the sized case neck too small. Or, if new case necks are too small or out of round enough that uniforming them to some mouth diameter is best for their use.

[higgite]

I suppose their could be times where you wouldn't need to expand the case mouth, where just chamfering the inside of the mouth would be suffice, but I haven't seen it yet.

Boat tail bullets.

[dickttx]

Expanding and flaring are two completely different things.
Expanding is to bring the inside diameter of the neck up to the proper size to allow a bullet to be inserted with proper neck tension. It has been several years since I have loaded rifle cartridges but all my rifle sizing dies had an expander ball that performed this step as the decapping pin/expander ball was withdrawn from the case. On a pistol die the expander is a separate die and usually includes a ledge that can also flare the top of the case if it is inserted far enough into the case.
The flare may or may not be needed but I would think that the expansion would always be needed.