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"M"-Provements On Lyman's "M" Die
by
MikeG

Neck expanders are a vital tool to the reloader. They are used to expand the necks of resized cases so that the cases will allow a bullet of the correct diameter to be started in the case mouth, yet not fall completely into the case. In addition, the case neck needs to exert some tension on the bullet, and the base of the bullet must not be damaged while being seated. Last, we'd like the expander to not stretch the case mouth too much, as this will lead to work-hardening the case and eventual failure, usually due to splits at the case mouth.

The typical expander takes one of two forms. An expander for bottle-necked rifle cartridges (.30-06, .223, etc) is merely a wide spot on the decapping stem of the resizing die. It is typically 0.001" - 0.003" less than the intended bullet diameter, which will allow for some case neck tension on the bullet and (usually) allow a jacketed bullet to be seated without damaging the bullet base.

An expander for straight-walled handgun cartridges (ie .38 Special, .44 Mag, etc.) has a different form and is usually not a part of the resizing die. This type of expander has a long, straight section that is 0.002" - 0.005" less than bullet diameter, and ends with a tapered section that goes quite a bit above the bullet diameter. In practice, the die is adjusted so that the case mouth runs up on the tapered section just enough to flare the case mouth so that a bullet can be started without damaging the bullet base. Too much flare and the case mouth will split, or in extreme cases might be expanded enough so that it will not even enter the bullet seating die! Cases need to be close to the same length or they will not end up with a consistent amount of flaring.

Lyman's "M" expander is neither. See first photo. It has a straight section that is under bullet diameter, and another straight section that is just over bullet diameter. This design is nearly ideal, if not completely perfect for cast bullets. The mouth of the case will accept a cast bullet without damaging the bullet base, and there is a slight 'step' on the inside of the case which helps hold the bullet straight while being seated. Case length is not nearly as critical as with a typical handgun expander die.

I have used the "M" die with satisfaction for years, and find it so useful that the conventional expanders of some of my die sets have never been used - I purchased the correct "M" expander before reloading even one cartridge.

When I started to reload cast bullets in bottlenecked rifle cartridges, naturally the first thing I did was get an "M" die of the correct size. However, as well as they had worked for handguns, I was having trouble with rifle cartridges. The culprit was case neck runout.

When resized cases were expanded using the "M" die, the necks were being pushed out of alignment with the case body, sometimes by 0.010" or 0.015". This is pure death to accuracy. The typical expanders for rifle dies weren't much better, only producing decent runout only when the expander ball was moved up as high in the die body as it could go - which prevented the decapping pin from working. The standard expanders don't open the case mouth for cast bullets either.

Thinking about the problem a bit, it wasn't too hard to see the culprit. The "M" die body is universal and much larger than the cartridge case. There are only two die bodies for the "M" die sets, one short and one long, and both measure about 0.520" inside. When the expander starts in the case mouth, there is basically nothing to keep it from pushing the case mouth to one side or the other. Whichever side of the case mouth was the weakest or thinnest was the most likely direction the brass would expand.

As an example, I resized some once-fired Remington .30-06 brass. With the expander ball removed from the resizing die, the inside of the neck of a sized case measured about 0.300". The smaller diameter of the .30 cal "M" die was 0.306". The "M" die has to open the case mouth from 0.300" to at least 0.306", without the body of the case being supported.

While this problem also potentially affects handgun cartridges being resized, case neck concentricity is not on the top of the list of things that affect accuracy when shooting a revolver with 6 different chambers, a cylinder gap and forcing cone that the bullet has to jump through, and iron sights to aim the gun with.

But on a scoped rifle I'd like to have my ammo performing it's very best.

Clearly, the case needed to be supported while the neck was being expanded. So, if it couldn't be supported from the outside like a typical resizing die, it would just have to be supported from the inside!.

Inspiration struck when I decided to carve out my own "M"-type expander. Having a lathe, and more time than money, I was partly doing this as a project to work on my machinist skills when it hit me that I could also customize the expander plug at any diameter I chose... and as many steps in the expander plug as I wanted.

Needing a .338 expander (which the gun shop might or might not have anyway), I measured the inside of a resized, but not expanded .338 Win Mag case. It was about 0.332"-0.333" (Federal nickel-plated). So, the first part of the expander is a pilot that just fits the resized case mouth.

Intending to use this for cast bullets, I decided that the next step should be exactly 0.338", so that a 0.339"-0.340" bullet will have just a bit of neck tension. The third step in the plug is 0.342", making it easy to seat a cast bullet without damaging the bullet base. The brass case will spring back a little so it's necessary to make each diameter slightly larger than what you'd like the final dimensions to be. See next photo.

My theory is that the smallest or 'pilot' diameter will help hold the expander and case neck in alignment while the rest of the neck is being expanded. Does it work? Does a bear sleep in the woods? According to my RCBS Case Master gage, concentricity of expanded necks were all within 0.004", and many were 0.002" or 0.003" (if the die body is perfectly concentric, the cases will measure 0.000" on the gage after being resized. My .338 sizing die - a Lyman - leaves necks measuring 0.001", which is very good for a run-of-the-mill resizing die that I picked up on sale). My typical results when using the standard neck expander ball, with ANY of my die sets, can range from 0.001" to 0.010" runout. Anything over 0.007" starts to be noticable on targets.

In addition to the nice concentric case necks that I was getting, I also now had the benefits of easy bullet starting and no damage to bullet bases. Tests are continuing, but it appears that using the "M" type of expander also helps seat both jacketed and cast bullets much straighter in bottlenecked cartridges. It certainly doesn't hurt to start out with the case straight to begin with.

In addition to the home-made die, I also tried modifying some of the extra Lyman "M" expander plugs that I had on hand. This was just a difficult as making a plug from scratch, for two reasons. One, the Lyman plugs are very hard. A file will not touch them - probably Rockwell-C 60 or greater. So a carbide bit is necessary. Two, the steps in the plug need to be kept concentric. Cutting a pilot step on an existing plug and keeping it concentric with the other diameters is quite difficult, unless all the steps are 'touched' with the lathe. Best way to do this is by starting out with the next larger diameter plug. In my case I modified a .311 plug for the .30-06 (see next photo).

One last suggestion, which will be my next project, will be a 4-diameter plug. Why 4? Well, the two modified plugs that I have are set up for cast bullets. Why not create a plug where it can be used for BOTH jacketed and cast? It is very little additional lathe work. For example, in the .30-06, the pilot diameter needs to be 0.300" to match the brass I have on hand. The next step up should be about 0.305"-0.306", which would give good neck tension for jacketed bullets. Third step should be 0.309" or so, which will give good neck tension for cast bullets AND can also be used to slightly open the case mouth for jacketed bullet seating, as desired. Last step would be 0.312" or so, to make it easy to start .310" cast bullets.

Each of the first three steps needs to be as long as the case neck. The last step only needs to be about 0.050" or whatever is convenient while doing the lathe work.

Questions? Shoot 'em to me at the Beartooth Bullets Shooter's Forum

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