Necking down cases one to three calibers will thicken the case necks. The amount of thickening is to some extent predictable by simple mathematics. Since the cases also extrude longitudinally the actual thickness is somewhat less than the calculated one. The amount of extrusion is also depends on the hardness of the brass. Prior to outside neck reaming you should take this into consideration and establish the exact amount on a test case. Note: Remove only enough brass to true the neck walls to start with.

This process is particular useful with expander plug dies, and used only for the neck diameter change. I am using as an example two factory loaded shells in 7mm Wby and 257 Wby. since somebody wanted to know how to neck down a 7mm STW to a 257 STW and had trouble pulling the expander plug from the necked down case. The length of the neck is not as important as the thickness, but should be close and the same for the smaller caliber. Also a shorter necks will have a proportional lesser extrusion.

In some instances the thicker neck walls are desirable for a better fit in the chamber. To take advantage of thicker necks bushing dies will do the best job. Before the 22-250 became a factory round it was called the 22 Varminter. Dies were made to allow for the neck thickening and nobody ever did any outside neck reaming. A 0.227" inside neck reamer would get rid of the neck thickening near the neck and shoulder junction, plus a trim cut, that was it. In a 22-250 factory chamber it still better to use the 250 Savage brass and fit the neck with a 0.0015 radial clearance for better accuracy.

The calibers for necking down are numerous and the number of steps required for some of them are not applicable to this simple mathematical solution but can be used as a guide. Here the intention is to provide a simple comparison, to neck down one or two or perhaps three calibers.

In this case we are necking down the 7mmWby to 257 Wby (or the 7mm STW to the 257 STM.). We start by measuring the OD of a loaded shell and deduct the bullet diameter to get case wall I.D. The loaded 7mmWby have an OD of 0.314" the inside is 0.284" the difference is 0.030" divided by two is 0.015", which is the neck wall thickness.

The mean diameter of the 7mm Wby is .314+ .284/2= 0.299"x 3.14=0.939 circumference. Multiply this by the thickness of the neck wall and the length of the neck and we get the volume of the brass in at cubic inches. 0.939" x 0.015" x 0.350"= 0.004929 cubic inches.

Doing the same with the 257Wby we get OD 0.285- I.D. 0.257= 0.028/2=0.014 = Neck wall thickness. Mean diameter = .257+.285/2= 0.271". .271 x 3.14= .851 circumference. O.851x 0.014 x .350"= 0.004170 cub inches.

This is the volume you need for the new case with a 0.350 long neck.

If we divide the 4170 by 4929 we get 0.8460 or 84.6%. So the 257 has 15.4% less brass in the neck then the 7mm. Meaning you has to reduce the 7mm neck wall thickness by 15.4%.

Now with the 7mm having a wall thickness of 0.015 we need to remove 15.4%. So we multiply 0.015" x 0.154= 0.0023" this is the reduction in neck wall thickness. However this reduction in thickness will be somewhat less due to longitudinal extrusion of the brass which also depends on the on the hardness of the brass. If an expander plug is used during neck down with out prior reduction of the neck wall, neck wall and shoulder stretching will likely occur trying to force 4-5 thou brass out of the way.

You could reduce the case neck walls by 2 thou, by outside neck reaming before necking down. This will allow your 257 expander plug to work as designed, and will preserve the concentricity of the new case necks. Needless to say the necks should be annealed after. If you have a matched .2572" expander die for the 25 cal neck reamer pilot, you can do the reaming after necking down. In this case you have to anneal the necks before expanding and reaming.

Don’t reduce the expander plug otherwise you get too much tension on the bullets, or when the necks thicken after repeated loading. As a wild cat loader you should be familiar with outside neck reaming. And don’t even think about inside neck reaming, for a parallel case wall reduction, because you won't be able to perfectly center the reamer.

Inside neck reaming is only used normally with fired cases which have thickened near the neck and shoulder junction. The fired case allows you to start the reamer partway into the neck, which self centers the reamer. In some applications inside reamers are used in conjunction with outside neck reaming. And please remember various neck wall thickness kills accuracy.

It is advisable to outside neck turn if only to have the neck walls all around the same thickness. It is not unusual to find case walls 1 ½ thou thicker on one side of the neck then the other. If this situation is not corrected at the start, repeated reloading with an expander plug will make the variance greater every time you reload the case. Because the case neck will stretch the most on the thin side and eventual the case will split at that point. Truing the necks is perhaps the first step in necking down and no wild catter should be without an outside neck turner and a 1" tubing micrometer measuring in 1/10000" if he wants a rifle to shoot straight.

Home