Tapered sizing buttons/expanding case mouths?

[Naphtali]

To expand cartridge case mouths, Redding makes a Tapered Size Button. This apparently expands a case mouth in one pass through die and press - in the sizes offered.

1. How efficiently does this device work - that is, what is the [estimated??] case loss from split necks? Any recommendations or precautions to reduce this case loss?

2. I anticipate other die manufacturers offer devices whose purpose is similar to the Redding. Those with first-hand knowledge with expanding case mouths, what devices or procedures are among the most efficient? Any particular things to monitor or do when doing this type of job?

[kywoodwrkr]

I've used the 7mm TCU configuration and am very happy with it.
I even bought up some others as they became available over time, just to have as forming aids.
I think other makers are now offering this option on some die sets.
I've questioned whether this wouldn't act as an M-die sort of in another die body.??
FWIW

[Doc Highwall]

It's only purpose is to expand cases up to that caliber and to straighten case mouths that have been dented. The best tool for expanding case mouths to accept cast bullets is the M-die. When you use a regular set of sizing dies they are designed to shoot jacketed bullets and the expander ball is made to give .001" to .002" press fit for a jacketed bullet, .308" bullet into a .306" to .307" case mouth. Now when you typically size your cast bullet .001" to .002" larger then the jacketed bullet you end up with too much of a press fit that will size down your soft cast bullet and also because of the extra force in seating causes run-out of the loaded cartridge. For my 308 Win I size the bullets .310" and my home made M-die expander is .309" Just belling the case mouth still leaves the inside of the case neck undersized taking away from accuracy. The Lyman M-die design has no room for improvement, only quality control in the manufacturing of it