.308 Bushing Die/Factory Chamber Questions


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LeonCarr
May 7, 2013, 10:46 PM
Hey Folks,

I have been thinking about making the jump from standard dies to either the bushing type sizing dies or benchrest type hand dies for my Remington 700P .308. In preparation of ordering stuff and to make sure I would be ordering the right size bushings, I took some measurements:

The necks of the fired cases coming out of my rifle have an OD of .345. With an estimated .001 of brass springback, this would indicate a chamber neck diameter of .346. After consulting with a buddy that builds precision rifles, a standard SAAMI spec .308 Winchester chamber runs .342 and when he chambers for turned necks the chamber runs .335.

I then measured the sized cases from my Redding FL sizing die and my Redding Neck Die and the sized case necks measured .330.

Here is where it gets good. Using Winchester brass and Sierra Matchkings I am getting a loaded round neck diameter of .333.

The manufacturers say to measure loaded round diameter, and use a bushing .002-.003 smaller. With standard dies, I am already sizing .003 smaller than loaded round diameter, but not really getting the benefits of less case sizing since the necks are being sized .015-.016 thousands of an inch due to the "Big" factory chamber neck diameter.

The question is in ya'lls experience is a .346 neck in a factory .308 Winchester chamber normal, and also for me to get the full benefits of bushing dies will I have to rechamber or rebarrel due to the big neck diameter?

Thanks in advance,
LeonCarr

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USSR
May 8, 2013, 07:06 AM
Hi Leon,

I have used bushing dies exclusively for years, so here is my take on this. First, your fired brass coming out of your rifle with a .346" outside neck diameter is normal. The brass out of my FN SPR comes out the same size.
Measure the outside diameter of a loaded round with the brass that you will be using. This diameter will vary according to the make of brass you use. In your case, with Winchester brass which is the lightest and has the thinnest necks, the diameter is .333". So, you want to have a .331" outside neck diameter after sizing. But, and this is important, you cannot use a .331" bushing and reduce your case necks from .346" to .331". If you try that, you will get results of .330" or less. When you try to reduce the neck diameter more than .005" with a bushing, it will reduce it at least an additional .001" from whatever the bushing diameter is. You have to reduce the neck size in steps when you have a great amount of reduction to do. For example, use a .340" bushing, followed by a .335" bushing, before finally using the .331" bushing. Or, you can use a .333" or .332" bushing and let the over reduction in diameter work for you that way. Hope that helps.

Don

Walkalong
May 8, 2013, 08:28 AM
The OD of the neck on brass fired in my FN SPR measures .346 to .348. Loaded rounds with a 168 SMK are .338 to .339 using Lapua brass with neck wall thickness of .0141 to .0150. I am using a .338 bushing (RCBS bushing - Redding Type S FL sizer) for light neck tension, loading to fit the mag at 2.800 OAL. Shoots very well.

emb
May 8, 2013, 08:51 AM
The diameter of your fired brass is irrelevant to the determination of bushing size. You can determine the correct size 2 ways. Measure the OD of a loaded or dummy round in the brass you will be using with a micrometer. Then subtract .001 from this measurement. This will be your bushing size. Ex. for 7mm OD .310 -.001 = .309 bushing. You can also measure the thickness of the neck wall, add the bullet and subtract. Ex. for 7mm (Neck wall = .013 x 2) + .284 - .001 = .309 bushing.

Go to the Website of the die that you plan on using and they usually instruct you how to choose the correct bushing. I know Redding has this under its FAQ section.

Good luck.

LeonCarr
May 8, 2013, 10:41 AM
Thanks for the great and detailed responses guys.

Lots to think about :).

Thanks again,
LeonCarr

USSR
May 8, 2013, 02:10 PM
The diameter of your fired brass is irrelevant to the determination of bushing size.

That is SO not true. If you try to reduce a neck size much more than .005" with a particular bushing size, you will end up with a neck size less than the size of the bushing. This was found out more than 10 years ago when bushing dies began being widely used by the guys on the Snipers Hide website. From Redding:

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".

Don

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