Norm Johnson

     The "M" die expands the case as usual, then puts a small step which is a couple thousandths over nominal bullet diameter at the very mouth of the case, allowing for straight and true bullet starting.

     A few years ago I purchased a dozen "M" die expander plugs from Lyman to use when I need an over-size or special diameter plug. These I have a machinist friend turn down for special experiments.

     They do, however, come in a great variety of sizes from Lyman if you do not have access to a lathe or machinist's services. They are listed below.

     (The plug screws into the top thing. The top thing screws into the expander body. The hip bone... Note that expander bodies

 come in two lengths. M1 is long (left), M2 is short (right). You can purchase various dimension plugs from Lyman. You can buy untold quantities of stuff from Lyman, they never seem to complain. If you have a long, or M1 expander body, you won't be able to expand short cases. I bought a 7MM "M" die, that came with a long or M1 expander body. When I tried to expand 7MM TCU cases, I found that even after screwing the top thing all the way down, I couldn't get the case far enough into the die to expand it. This led to confusion. A call to Lyman cleared the whole thing up. I needed a SPECIAL 7MM TCU "M" DIE. I bought it, and could then expand the 7MM TCU cases. You can expand long cases in the short M2 expander body. So if you have one = 1 short M2 expander body, you can screw any plug into the top thing, screw the top thing into the short M2 expander body; and expand away. joe b.)

     How are these dimensions related to the 'steps' above?

     The main shaft of the expander portion is the "First Step". It is typically a few thousandths smaller than bullet diameter, as are conventional expander buttons.

     The "Second Step" puts a small step of increased radius in the case mouth about 1/16th inch deep (adjustable). This step is a few thousandths over bullet diameter to aid effortless starting.

     What are expander bodies?

     The die bodies that hold the expander plugs; directly analogous with conventional expander dies.

See the Lyman catalog (or above) for a picture which is worth a thousand words.

     How much expansion or bell does the M plug put on the case mouth?
Ideally, for lead bullets, the first step would be about .002" under bullet diameter. For jacketed bullets, the first step should be .003-.004" under bullet diameter. In either instance, the second step will be 2, 3, or 4 thousandths over bullet diameter.

     Usually the best lead bullet diameter is about .0005" less than the chamber's measured throat. Most of my .30 caliber rifles shoot .310" or .311" cast bullets best. I keep several plugs on hand and have a machinist friend turn them to my specs if I think it will help.

     It is also highly desirable, on a two diameter rifle bullet, for the nose of the bullet to be of bore riding diameter. To check this for a given bullet, push the bullet nose first into the rifle's muzzle. If it goes in without resistance, the bullet nose is too small and good accuracy is not as likely.

     Does the seating die crimp out the bell or do you just leave the cases expanded? Your choice. For my
lever actions I Sometimes crimp. For bolt actions, I do not.