Works pretty nicely for using cast bullets. Helps prevent shaving lead off the sides...
also nice is that it expands the case evenly, deep enough to seat the whole bullet without forcing it through a tightly sized case. A flaring die only expands a short section of the case, which allows the bullet to start, but then tightens back down. This can deform a soft lead bullet. The m die expands down the full length of the bullet, allowing more consistent, concentric seating.
It's another tool to aid in consistent reloads....
I think they are still available.... if you have access to a lathe, you can make your own inserts. I have one die, and made inserts for .22, .30, etc... you can turn down either a bolt, or an all-thread to whatever diameter you want.
I don't remember the bolt size... maybe 3/8" I'll have to check.
You might find one on fleabay, who knows?
The Lyman M dies are readily available. Here's a tip - there are only two sizes of actual dies, Long and Short. If you have both dies, you can order whatever expander plugs you need direct from Lyman, and interchange them as needed. That becomes tedious, though, changing the expander and adjusting it every time you switch cartridges.
NOE (Night Owl Enterprises) offers expander plugs that work with the Lee Universal Expander Die. They offer at least a couple of different diameters for each caliber. NOE also makes excellent bullet molds and other stuff.
If you already have the Lee Universal Expander Die, you could just order the NOE expander plugs in the size(s) you need. I don't think that they're compatible with a standard Lee expander die.
I do have the LUE die. I just need some range time to see if I have a leading issue.
I also need some loading time to up my charges from bunny fart status! I started at “Will it cycle/fit” for auto/revolver and ready for “Will my powder oat hood together?”