ALL cases will benefit from M-Dies. Its NOT just for cast.Gas Checked Cast Bullets seat just like a Jacketed bullet. It just seems a little more slippery going in, and it's a lot harder.
EVERYTHING with an accurate load is consistency, 80% of an accurate load is neck tension.
But Ian that would be improper use! How can you condem a product for improper use?That's why you don't want to get too crazy with that second, .004"-oversize step on the Lyman and NOE M-dies. If you drive that second step down into the neck more than the absolute minimum to create a bell, you have positively ruined the neck tension in that area. The stepped expanding spud design is a reloader's convenience, not an accuracy boon. I think a lot of people make a stepped neck so they can slip their bullet down in there and it be held straight while the bullet is seated, but all that does is effectively obliterate neck tension where it is needed most. If you are resizing with conventional dies and just using the larger M-die step for "belling", it might not be enough....
I get it that those who "know what they're doing" don't over-do that second step, but still, the transition between the steps is abrupt. The RCBS spuds have a much more gentle taper and ease the bullet base into the neck without having to jam it over an abrupt step.
Apparently the RCBS Cowboy dies have a much more sane transition between the two diameters. Still, nothing wrong with a bellmouth unless you're using a .354" spud to load .3585" plain-based cast bullets, and that's what my .357 and .38 SPL spuds measure.
And this is why a mold I sent to someone on another forum could not get any accuracy out of his rifle. I am sure he uses M-dies. I am running this same bullet at 2200 fps in my Mosin and he lost all accuracy at 1900. I use the RCBS neck expanders after Ian told me about them when I first started loading for 30 cal rifles.Because Lyman doesn't make the correct sizes for the correct neck tension for HV .30-caliber (and several others), and as I said, the angle of the transition between diameters is too abrupt to ease transition of the bullet into the neck so your bullet bases have to bump over that step. You're stuck with two sizes for .30 and that's it. The abrupt transition can be fixed with a lathe or drill press and file, but you can also turn around and buy a spud of the correct size and belling taper from....RCBS. If you don't get that neck tension just right, and consistent, you'll be just like 99.5% of cast bullet shooters who think 1900 fps is the accurate limit with cast bullets in their favorite .308. Lyman tooling is designed for something other than what I need or do, so I buy from those who offer what I need or make it myself. If their product works for you and plain-based revolver bullets, fantastic. RCBS's standard pistol die expanders are too small for cast bullets as well, so I'm not saying they're perfect either. At least RCBS offers die sets made for cast bullets that actually have the right sized parts in them.
If you happen to find an M-die that's made the correct size for what you need (their .44 caliber is almost correct for tight .44 Magnums using .430" bullets), then you're good to go. Otherwise, have fun working with tools that make things wrong. IIRC, reading Lyman's own website long ago, they recommend their M-dies for loading jacketed as well. Don't get me started on how much resizing dies over-size the brass in the first place.