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>> "M"Proving On The Lyman "M" Die Part II :: By Mike Giboney on 2003-08-22
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"M"-Provements on Lyman's "M" die
Part II

After the success of the first modifications of Lyman's "M" expander die (see "M"-Provements on Lyman's "M" die), further applications of the design were discovered. The pinnacle of standard "M" expanders ended up being a custom "do-all" expander which covered cast and jacketed bullets for .375 cal rifles. The .38-55 in particular has a reputation for widely varying groove diameters, and so an expander was conceived with extra steps - the reloader merely adjusts to get the case mouth expanded to the desired size. See photo below:

The pilot section on the nose of the expander (sized to just fit into the mouth of a resized case) did an excellent job of controlling case neck concentricity or runout, by holding the case neck in position so it could be uniformly expanded. It was discovered that bullet seating was also made much more precise, as long as the final 'step' in the expander closely matched bullet diameter. Case mouths spring back on average 0.002" so for jacketed bullets, the top step would be 0.0015" - 0.002" over actual bullet diameter, which allows bullets to be started by hand with no 'wobble'. For cast, protecting the bullet base is more important than concentricity so a slightly greater allowance of 0.002" - 0.0025" over actual bullet diameter works very well for opening the case mouth. The majority of the neck is left under bullet diameter for the required case neck tension.

However, as useful as the design worked out to be, it is limited to neck-expanding only. There are times when full-length resizing of bottleneck rifle cartridges is either necessary or desired. Having achieved outstanding results with the M-die modifications, I began to search for ways to apply this to standard (ie. RCBS/Hornady/Lyman etc.) sizing dies.

As it turned out, many of the benefits of the pilot section can be applied to a custom expander. A typical expander is a wide spot on the decapping assembly, as seen below in this RCBS decapping/expanding assembly from a .257 Roberts sizing die:

There are several problems created by this design. First, the case neck is not really supported very well during the expanding operation. It can expand in whatever direction that the brass is thinnest/weakest. Second, this example shows a parallel section for expanding which is really too long, creating excess drag.

A new expander design was then created which would address these issues. First, a pilot section was turned on both ends of the expander button. Second, the section of the expander button which actually expands the case neck was made very short, which cuts down on drag. The following picture is of a custom assembly made for a Hornady die. Note that the Hornady assembly is different in that it uses a collet to hold the expander in the die, and also that this example does not have a decapping pin, as it is designed to be placed as high as possible in the die. However, the design principles can be applied to any ordinary expander button.

This design was found to lessen the typical drag/chatter experienced when an expander button is drawn out of a case, even when the button is in the 'low position' (near the bottom of the die). Still, it seems impossible to eliminate all drag/chatter in the low position. Perhaps this expander design in either carbide or with a super-slick coating of TiN (titanium nitride) would be perfect.

In the high position in the die, however, this works to near perfection. Very little drag and does a wonderful job of keeping case necks concentric. One further, very unexpected benefit was also discovered.

To appreciate the benefit, first the problem must be fully understood. The first obstacle to case neck concentricity is the sizing die itself. If a case is full-length sized, without the neck being expanded, the outside of the case is a near-exact representation of the interior dimensions of the die. Should the die neck be concentric with the die body, then a concentricity gage will read 0.000" (in the real world 0.001" is actually very good). But... if the neck is not perfectly aligned with the die body, and they usually aren't, you might see a reading of 0.003" and up.

I have a number of sizing dies on hand. The very best is a bench-rest style neck sizing die which leaves cases almost perfectly concentric, the only reading on the gage coming from surface imperfections on the case necks (less than 0.001"). Another .30-06 full-length die consistently reads 0.002" - every case. While not bad, it's somewhat frustrating to know that I can't do any better than that - or at least I didn't think that I could without replacing the die.

For that particular die, I made an expander button with my new design, and put it in the die in the 'high' position. Running a few cases through showed an average runout of 0.003" - 0.004". Not terrible, but not as good as I expected. It seems like the die button wasn't perfectly round, and I considered making a new one.

Then it suddenly occurred to me that perhaps the imperfections in the button and the die body could cancel each other out. Indexing the button a quarter-turn at a time, I found a setting where cases now consistently read 0.001" of runout - BETTER than the die should be capable of. Needless to say the expander has been locked in that position and isn't coming out of that die for any reason!

Thinking this must surely be a fluke, I found a sizing die which was even worse and got some improvement out of it also, by tinkering with the expander position. This is definitely an area that is worth checking in the quest for maximum accuracy.

Other than the theory that the button may be slightly out of round, there is one other possible explanation for this behavior. Having the button high in the die body may give it better leverage to 'straighten' or 'push' the neck back into alignment. In the low position in the die, the long decapping stem isn't rigid enough to be able to move the neck much without the case body being supported, especially in the small calibers.

Lessons learned -

Even without having the facitilies to create custom expanders and such, there are some things that can be done with standard reloading equipment to improve handload quality. A case neck concentrity gage can be very helpful in discovering the source of problems with reloading equipment. I once found a cracked bullet seating stem when trying to figure out why I was having problems with a die set. Checking concentricity of cases that have been full-length sized (but not expanded) will give some insight into the potential of your dies. Then, experiments can be made with expander button positions to keep case necks as straight as possible. As noted above, I have saved at least one die set from being replaced. That alone recovered nearly half the cost of the concentricity gage!

Also, neck sizing dies sometimes produce worse results than full-length sizing dies. This can come as a shock to the handloader who has been brought up on the notion that neck-sizing always produces the greatest accuracy.

A concentricity gage is like a chronograph, once you have one you'll wonder how you ever got along without it. It can definitely pay for itself very quickly in components saved for the accuracy-minded reloader. In addition, there are definite improvements that can be made over standard reloading dies and equipment for those who take the time to understand how the pieces work together and what modifications are useful/necessary.

Probably the biggest benefit that I have obtained from all of these experiments has been how to get better ammunition from my off-the-shelf reloading dies. Some of them have been purchased for ridiculously low prices as used equipment at gun shows, etc., and yes, I am quite pleased that I can sometimes get bench-rest quality ammo from $10 die sets!

Questions? Shoot 'em to me at the Beartooth Bullets Shooters Forum ( ).

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