Originally Posted by
geargnasher
The goofy steps on the M die is the problem for cast. They work great for jacketed bullets.
To load truly accurate rifle ammo, case neck tension is rather critical, as is not deforming the boolit or the case neck during seating. This is the whole issue with the M expander, it doesn't facilitate either of these things because the design is flawed. I'll explain:
If you use the boolit to stretch the case, you are messing things up, and the M die requires the boolit itself to do the expanding of the neck below the first step in the mandrel. Cast boolits tend to get squished slightly and bent when this happens, and the necks can get bent or deformed too. Only slighly, you can't see the difference, but it's there and it's usually enough to get the boolit started crooked.
Even and consistent case neck tension is the other bugaboo. Getting right at .0015" neck tension on .30-caliber cast boolits seems to work the best for me. One thing I've also noticed is that the brass has a tough time "changing directions" when stretched and compressed. You size the neck down (hopefully no more than needed), then reverse it and expand it back up. To change the shape/size fo the neck the brass must obviously be pushed beyond it's elastic limit. It takes considerable force to reverse the direction the metal moves and expand it, and you want to do this with a steel expander, not the boolit. Thing is, most rifle brass has about a .002-3" elastic limit depending on state of anneal, and the M dies have the "primary" step machined smaller than that. For example the .30 caliber one has a .307/.3105 mandrel and only expands a short part of the case to .3095 at the most after springback. If you're using a .310" boolit, it's having to stretch the .307" part out by forcing the gas check in there, and the brass is right at it's elastic limit. For some cases, it will be beyond the elastic limit, for others even in the same lot maybe not quite up to the elastic limit. Riding this ragged-edge makes for very inconsistent boolit tension, hence the reason I hate the steps. If you're going to make a stepped spud, which is a defective concept, at least make the step only about .0015" or so, so that the brass will be properly stretched and conditioned to function WITHIN its elastic limit when you seat the boolit.
The ideal expander spud for your particular gun and boolit size will work in conjunction with your sizing die. Size the neck down to where the ID is a couple thousandths below the diameter you want it before seating the boolit, then stretch the brass the other way with a good, parallel-sided expander that's the right size to give you about .0015" smaller than your sized boolit diameter after springback. The expander needs to be long enough to pass clear through the neck and expand the whole thing, and expand it straight in line with the rest of the case. It also needs to bell the mouth a bit so you can start the boolit, and this is an often overlooked thing with expanders: If the bell is part of the expander, it will open the mouth the same way all the way around. If you use a Lee Universal expander or one of the really short pistol PTE expanders, the flaring part isn't guided and centered in the neck by the expander mandrel and thus can stretch the mouth of the case more on one side than the other depending on the condition of the brass, which mind you is never as perfect as we'd like it to be. Having the bellmouth shank doing its thing while the expander spud is keeping it centered and piloted the full length of the neck is important. The M die does do this part, but in many calibers they don't bellmouth properly.
Now, "what about the parallel part starting the boolit straight?" you say? If the case neck is prepared to the correct dimension and tension, the boolit will go in there straight, because straight is the path of least resistance. If the boolit is having to stretch the brass past its elastic limit like is usually the case with M dies, it can push it any direction it wants because the resistance is the same, hence crooked cartridges with all sorts of runout and uneven cast neck tension on the boolit. The other thing I don't like about the M die is that after seating the boolit, you have lost fully a fourth of the holding surface of the case neck because you stretched it to the point it barely touches the boolit. If you think about it, that's sort of a dumb thing to do. If you use an RCBS or similar expander on your rifle case necks, they will grab the boolit with equal tension from the gas check to the base of the flare, and the flare takes much less length than the "starter" portion that the M die creates. I realize you don't necessarily have to push the second step very far down into the case, but the way they're designed, most of the time you have to to get the mouth open enough to start the boolit.
So there you go, it's all about proper neck tension, straight necks, and not deforming anything when you seat the boolit because the boolit is only springing the brass open a small amount and staying well within the elastic limit of the brass when seated. You obviously need to co-ordinate your sizing and expanding dies to work the brass a minimum amount, and often you must segregate your brass between rifles if you're really splitting hairs.
Many of you will argue with me on this and say "I've use and M die for 50 years and never had a problem". Well, fine. If you're happy with what you're doing, then it isn't a problem for you. But for some of us chasing accuracy and higher velocities, these things start to really show up, and the M die won't get you there most of the time.
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