Let's have that discussion.
It is commonly and erroneously repeated that the M die does something "special" for cast bullets in terms of preparing the case neck inside diameter. It does not, in that it does not "specially" size the case neck compared to the Hornady expander CDOC mentioned. Both are intended for JACKETED bullets, and size the case so a .458" diameter jacketed bullet is gripped correctly. When speaking of .461" bullets, the neck tension is "tighter" than needed, and the M die does nothing.......repeat, nothing the Hornady expander does not do. If the bullet is soft and oversize, both may swage the bullet to some degree since they are intended for jacketed bullets. Harder bullets may avoid the effect, as will gaschecked bullets. Both are intended for jacketed bullets, but may be adapted to cast bullets due to another feature.
What both the Hornady and M die do is flare the case neck to accept the cast bullet heel without shaving, but the M die doesn't do it any better than the Hornady die does, as it flares the case neck as well.
Some also erroneously proselytize for the M die in that the step allows "straighter" bullet seating, but this is also a misconception. The same straight alignment to start can be had by simply giving the nose of the bullet a gentle tap against the seating stem before giving it the full seating stroke (withdraw the case after gently tapping it and before fully seating the bullet and you will see the bullet is aligned with the case using this simple expedient). Truly straight bullet seating is not had due to the M die's influence when the heel of the bullet is merely 1/16th inch in the case neck, because the seating die is responsible for guiding the bullet the rest of the way. So the seating die's alignment with the shellholder and cartridge/bullet and the fit of the seating stem to the bullet's nose is responsible for straight bullet seating more than the illusory claims for the "step" in the M die.
I do not have a Lee expander for 45-70, but I am aware they are on a continuous taper that gets larger as it goes up. So, make sure the flare is adequate before seating the bullet, Doc, and likely your problems will go away. Check to see the heel of the bullet can be fully admitted into the case, trying it by hand, before putting it in the seating die.
Doc, measure the diameter of the Lee die's plug at the bottom and top and report on this thread what they are.
All....and I mean all....three die sets for straight cases from all manufacturers have some means to flare the case neck, and this isn't exclusive to the M die. Witness RCBS, Redding, Lee, and other brands of pistol and straight case rifle dies. All will accept cast bullets. I seem to be loading 45 ACP's and 32 Smith and Wesson Longs and 38's in these other brands with cast bullets just fine, and there isn't an M die in sight, nor is there a need for one.
In any event, CDOC specifically asked about the advantages of the M die over his Hornady expander. There aren't any. Zilch.
Incidentally, when speaking of bottleneck cases, far better results may be had in terms of fitting oversized cast bullet diameters if the Lee Collet Neck Die is used. This allows adjustable neck inside diameter to fit oversized cast bullets to allow gripping them without any possible swaging down of diameter in a neck sized for jacketed bullets.