You cannot measure the size of the bushing, by measuring the neck OD of the sized brass.
Some years ago I got a really neat 300WM 40XB heavy barreled action that was unfired. It has a 12 groove barrel. I dropped it into an H-S M24 stock and it shoots like a house on fire.
The odd thing about it, is that the chamber is tight (minimum body), but the neck was large (??).
So the fired cases let a bullet easily drop in the mouth (with about 8 feet to spare).
I figured out what bushing I needed, based on the old formula of "desired loaded OD, less 0.002", which we all have been using since forever.
When the bushing came in, I neck sized all my fired cases in my nifty new "S" neck die.
I only sized 3/4 of the neck (the seated bullet contact area), and the necks looked really "Stepped". So I measured them and they were ~4 thou smaller than I had planned them to be.
I figured that the bushing must be wrongly marked, and took it out of the die, and ran a soft lead plug through the bore (the ONLY way you can measure the size!!!).
The bushing was proper size... as marked ??
So I dropped it on the sized neck, and it was
LOOSE.... damn!! In fact, it was so loose, that it was floppin' around on the neck of the case it had just sized!!
(more
)
So I got out a bunch of fired 30 cal cases from different 30 cal rifles I had at the time (four 308's, the 300WM, and a 300 wildkat), and used the same 300WM bushing on all of the cases, by moving the bushing to the relevant sizing die... and I played with them the rest of the weekend, and here's what I found.
The amount of reduction of the neck is NOT automatically one or two thou over the bushing bore dia, after sizing.
It depends on two things.
1- hardness!
2 - The size of the neck when you started out.
The first should be obvious to most of you... but the second works like this.
If the fired case neck is case is only 1 or 2 thou over the desired size, and the bushing is two thou under the desired size, the neck has only 3 or 4 thou distance to travel... when it hits the rounded edge of the bushing, it moves slightly and follows the contour of the bushing wall.
But if the fired neck is 10 thou, and you have to move it 12 thou,, the brass mouth flows into the bushing at a ~45 degree angle down the sides of the rounded edge, into the bore of the bushing, and when it gets to the inner dia, it "OVERSHOOTS" the bore dia, and keeps on going a few thou
well past the bore dia.
So that Monday, I called Patrick Ryan at Redding and told him about this mysterious thing, and he said he never head of such a thing. So I said that 99.9% of his bushing customers were using tight neck cases (there were no "no turn" chambers then)... and they were moving the brass only a few thou - not very much, and that is why he never heard of it.
We "politely" kinda disagreed about it, and the conversation ended with him saying that it was impossible.
About two hours later, he called me back and said, "Well, I'll be damned, I just did what you said!"
About a year later, the description of this showed up in their literature, and on their website.
For a full description, go here.
http://www.redding-reloading.com/tech-line-a-tips
At the bottom of the page on "bushing selection" it says:
It has come to our attention through customer calls and our own use of the bushing style sizing dies that in certain instances, a given neck sizing bushing will produce a case neck diameter that can be several thousandths of an inch smaller than the actual diameter of the bushing. This idiosyncrasy occurs when the neck diameter of the fired case is a great deal larger than the diameter of the neck sizing bushing, such as occurs when factory chambers are on the large side of the tolerance range and the brass is on the thin side. Typically, we have not noticed any problems until the case neck is reduced more than 0.008-0.010".
Solutions include, increasing bushing diameter to compensate and/or the use of a size button. Reducing the neck diameter in two smaller steps by using an intermediate diameter bushing will also help. More concentric necks will also result using this method, as the case necks are stressed less during sizing. Don't forget to properly chamfer the inside and outside of the case mouths and apply a light coating of lubricant to the case necks before sizing.
Now... if the problems can cause a neck to be so over sized that you need a bushing that is LARGER than the finished dia you want, it should be obvious that the sized neck dia is NOT the way to measure the bore dia of a bushing.
Push a soft lead plug through the bushing and measure it - anything else, and you are just kidding yourself.
Meow
