Interesting- I will look into this "M" die... sounds about right.
M dies are the Wal-mart level approach to giving your cast bullets their best start in any caliber. They are really only intended to make the theoretical cast bullet of that caliber slip into the case neck better. I used them for years, still have them, for some rifles still use them with cast.
There are (at least) two better approaches to match your cast bullet to the case neck you're going to seat it in.
- A Lee Collet die with a collet made to your spec by Lee's custom shop to obtain the amount of interference fit you desire.
- The previously mentioned neck expanding dies offered by NOE. You can buy them in graduations of .001" - again, to get the amount of interference fit you are looking for.
To get best accuracy in any rifle with cast bullets, it is no theory that matching bullet profile to the ball seat/leade is where you'll find your best accuracy for more rifles. You will not find many serious competitors in CBA benchrest competition who don't use either custom moulds or custom sizing to get the closest fit possible between the seated bullet and the ball seat/leade of their rifle's chamber. At the high end, they use a swage to swage the cast bullet in a replica of their chamber before seating the bullet. Some of the old Schuetzen match shooters actually pushed the bullet into the ball seat/leade prior to loading the charged case - essentially swaging the bullet to a perfect fit prior to firing.
If you spend a few minutes thinking about it, if your seated bullet has space around it for the time that it moves from the case neck until it jams into and seals the bore while pressure builds behind it, during that time the expanding burning gases have an opportunity to blow past the base and sides of the bullet at high temperatures and pressures. Which leads to gas cutting. Which also leads to depositing bits of lead down the barrel.
Fitting your bullets to your rifles' ball seate/leade is not rocket science. And once you've determined the proper fit, it is no harder to replicate with your cast bullets from that point onward.
First you need to determine your ball seat/leade dimensions by either doing a pound cast or using cerrosafe as described earlier. Then you determine the dimensions of that cast. I, like most who do this, look for the measurements in the chamber just in front of where the neck begins of the case portion of the chamber (bullet diameter + thickness of brass cass neck), through to where you have the bore diameter where the rifling first shows itself to be fully formed - if you plan on using a bore riding design, the front bore riding portion of your mould has to be about a thou less than the bore diameter it will be sitting within.
Then choose the design you think will work for you within those dimensions - and fit as best possible into the dimensions you now have for your chamber. I look for a design where the base of the bullet when seated in the chamber will close to the junction of the shoulder and neck, rather than intruding into the case space. My bullets are just touching the lands when the gas checked base is right at where the shoulder transitions to the neck. Take care of the little things and the big things will sort themselves out...
At this point, I find the maximum diameter for your cast bullet shank where, when seated in your brass of choice, the loaded round just chambers. You'll know this point because if the bullets are slightly bigger in diameter, they necks are too large with the bullet seated and they can either not be chambered, or require some force to fully seat them in the chamber. Not being a competitor in CBA any longer, I want rounds that smoothly chamber without any drag.
For the little extra price that custom moulds such as those from Accurate Moulds cost, in my opinion they are a no brainer. His moulds will cast to a thou or less at all parts of the casting to the dimensions you specified to him with your order. Second advantage: you can have him cut two (or three) different cavities to your specifications for not much more money: three different designs to try for not much more than the cost of one. Once I find the design that delivers the best accuracy, that first mould with those different cavities gets sold, and a new one with the dimensions of the best design gets ordered. Really no different than money spent testing jacketed bullet designs from Sierra, Hornady, Barnes, etc to find the best accuracy - and not really any more expensive.
I don't obsess with a perfect interference fit because you're getting close to the point of turning case necks for perfectly uniform dimensions at this point. I can't be bothered; in even the best of Lee Enfields, modern hunting rifles, etc, the error budget of the rifle is much greater than what is lost because you don't have that last .0005" of fit in the chamber.
Once you've figured out the amount of seating with the amount of sizing, you just replicate that as you size your bullets as you always would going forward (if you don't just shoot them as cast).
I prefer the NOE neck expanding system for a couple of reasons:
First, they are relatively inexpensive and are used inserted into a universal Lee die.
Second, that means buying a few in .001" increments so that you can see if differences in sizing have an accuracy sweet spot is cheap. I start out with the Lee Collet die to get the most out of my brass, but their custom sized collets made to your specs cost more than the original die. I am not rich enough to be able to buy custom collets from Lee in .001" graduations.
NOE also offers a very cool bullet sizing system where, once again, the individual dies are relatively inexpensive and can be purchased in .001" increments. You can also choose to just size the base, or just the nose (i.e. a bore riding design that is a hair too big as cast), or you can go full crazy and size the base and then the front portion. My SAECO lubrisizer doesn't do much other than square the bases and crimp on the gas checks these days for some calibers, in particular the Long Branch Lee Enfield and a Winchester 1895 in 30 US that has a .303 British size land and grooves barrel.
Again, matching the cast bullet profile to the chamber/ball seate/leade profile is not all that hard. It's a one time pain in the arse of getting a casting, then measuring dimensions along the casting for the portions where the seated bullet will be, and then picking a bullet and figuring out how you're going to go about getting a tight fit. After you figure that out and what sizing you'll have to do and how much... you just replicate that set up with each batch of cast bullets you cast and then seat.
