I use my M dies for a lot of case belling, but I don't buy complete dies for each use. I have two M dies and one is short, and one tall. I simply order the expander plugs directly from Lyman and install whatever plug I need for a specific cartridge. They're only $6 for the expander plugs and threaded body. I toss the body in a drawer and just thread the plugs into mine.
I think any 7mm plug wont be good to bell a .28 caliber case. I'd use a .30 cal. and set it as you would to bell any case and just touch the mouth.
My two M dies came to me used and a previous owner had milled the sides of the threaded areas open. So I can look through the openings to visually adjust depth. But even without the openings the dies are easy to set; just takes a little more time.

Question (imagine that)
Kinda like throat, leade, free bore,...……
The M dies usually perform a two step process. Expands case to just under bullet size and expands a tad of case mouth for easy insertion of the bullet. Is the second step belling? 
Screwed in farter the M die will "bell" the mouth in addition to the expanding. But why would you do that?

Red Cent wrote on Sep 9^th, 2018 at 2:21pm:


The step on a Lyman M die plug isn't designed to bell. And it has such an abrupt edge on it the bell wont be the usual nice gradual bell. I never use the step on the plug to bell, but rather use a size larger to bell case mouths.
I have quite a few M die plugs I've turned the step off of so I can run the expander plug in deeper and not have to stop at that step. It just gets in the way for longer/heavier bullets.

I use the Lyman because I have a Lyman, and the expander plugs can be purchased cheap from Lyman. It's very easy to spin them in my drill press and take the step off, or if one wanted to modify the step to make it a belling step.
But since I almost never expand case necks, unless I'm forming brass, I use my M dies almost exclusively to bell case mouths.