NECK SIZER DIE vs M DIE

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  • Last Post 15 September 2023
4895 posted this 14 September 2023

WHATS THE DIFFERENCE BETWEEN NECK SIZER AND M DIE

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Tom Acheson posted this 14 September 2023

Maybe one difference….

The neck sizing die only works the neck section of the case. The full length sizing die works the entire case, including the neck. Some shooters only use the neck sizing die after the full length sized case has been fired. In our low pressure cast bullet world, you can do this quite a few times before you need to full length size the case again. After quite a few multiple firings, you may have to full length size the case.

This practice is OK when using the same rifle for each loading. If you use the same chambering but switch rifles, you may have to full length size again.

The Lyman M-die slightly flares the case mouth, to aid in seating the cast bullet, without forcing the bullet into the case, resulting in shaved lead from the perimeter of the bullet. The amount of flaring varies depending on each combination of bullet and case mouth.

One way to aid in seating a cast bullet without using the M-die, is to neck size the case and deburr the inside of the case mouth with a VLD case mouth deburring tool. The resultant gradual angular or tapered profile of the inside of the case mouth might permit the bullet to be “easily” seated without damaging the cast bullet.

Hope this helps!

Tom

 

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Aaron posted this 14 September 2023

The M-Die provides a sort of cup with a rounded base for the cast bullet to set on before seating. It's better than a flared mouth like we use for handgun loads. It's not as exaggerated as the image shows but it gives you an idea of the mouth, waiting to set the bullet into. I use the M-Dies a lot with cast rifle bullets.

The M-Die is used after the neck has been resized by the sizing die to provide the cup for the cast bullet.

With rifle in hand, I confidently go forth into the darkness.

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Tom Acheson posted this 14 September 2023

One thing to consider if using the M-die. The case mouth “lip” that is expanded to be larger than the starting neck OD. Depending on the gun’s chamber, that lip may need to be “closed”, using a crimp die, so the lip does not prevent chambering of the round. I have several M-dies but in recent years use only one. That’s for a black powder cartridge rifle. All smokeless loads use a neck ID treatment as previously described.

Tom

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Aaron posted this 15 September 2023

Good point Tom!

With rifle in hand, I confidently go forth into the darkness.

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mashburn posted this 15 September 2023

the M-Die only expands the case neck to the diameter of the bullet. Hence, if you buy a .308 M-Die it will expand the case neck to .308 and the bullet will easily push down into the neck. That allows the bullet to enter the case neck in perfect alignment and makes bullet seating much more precise. You can adjust how deep in the neck that you prefer the bullet to go in. These things work, in most rifles. The problem is with shooting cast bullets, is that you have to have a die that expands to the size of what-ever bullet that you are shooting. The were designed for jacketed bullets which are of standard size. I guess you could make a new expander rod for each diameter of bullet that you shoot. I have some rifles, in which I fire jacketed bullets, have had their groups cut by almost half after using one of these dies.

It will not enlarge the case neck, after loading the case neck will be whatever it is supposed to be.

Mashburn

David a. Cogburn

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gnoahhh posted this 15 September 2023

I use an M die and an RCBS expander die. Frankly, I prefer the RCBS tool because I can more easily make my own custom diameter ones because of its straight-to-tapered flaring versus creating that little lip of the M die (plus I don't mind a simple flared case mouth, I remove it via straightening with seating die). I strive for a mere thousandth or so smaller than bullet diameter, even for revolvers because I crimp 'em.

Or (he say's sorta tongue in cheek), ditch the reloading dies and go straight to breech seating the bullet separately from the charged case. No resizing, expanding, seating, getting the bullet started crooked, etc, and one case lasts a loooong time.

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MarkinEllensburg posted this 15 September 2023

The best expander die that I have and use is one my dad made for .308. It is an old die body with a large cut out and a tapered expander that is polished. In use the die is mounted so I can see the expander rod and then that is adjusted with a case until a cast bullet will almost start. It requires cases to be fairly uniform. With the window it's easy to see if a case is too short.

I am in the process of making an expander for a fellow member. It will be step tapered to work with a range of diameters for a nominal .25 caliber. It will be the same basic design and I plan to make a post to show it off before sending it out.

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MP1886 posted this 15 September 2023

I'm going to point out some things wrong with the M Die. Bullets require neck tension, cast not as much as jacketed.  If you have a short neck cartridge like say the 300 Savage, 300 Win Mag, even the 7.65 Argentine, 223/5.56, and there are others, you end up with very little neck to put tension on the bullet. Contary to popular belief M Dies do damage base of the bullet some especial with plainbase bullets because that little "ledge at the bottom of the "cup" boogers up that sharp edge. The best expanders out there are the RCBS type.  One doesn't have to flare the case mouth that much, only enough that the base starts in it.  Far as seating bullets straight I use inline bullet seaters.  I believe M Dies got popular because you can have your cases in the loading tray all ready for bullets and the bullet fit up in them nice and don't fall out!!  One reason boatail jacketed bullets are nice to load. Lyman put out all this propaganda about the M Dies and everyone thinks they are great.  

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Tom Acheson posted this 15 September 2023

Yes, that variance in bullet diameter from the nominal (jacketed) diameter, when using cast bullets, can be a challenge. When I was shooting a lot of .30 cal. cast bullet rounds, they were often sized to 0.311”. Although 0.003” oversize seems small, it is a critical dimension for obtaining optimum accuracy. So it introduces the need to be creative is solving the bullet seating detail.

Tom

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