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Neck sizing

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Unread 09-11-2004, 09:28 PM
Bronze Member
Join Date: Sep 2004
Location: SC
Posts: 48
Neck sizing

I have been told some where that if you have a fired form case and you can put a bullet in the neck of it and it does not touch the side what so ever that you do not have to turn the neck.Is this true?
I have a RCBS case master gauging tool but i just got to get more info on this subject.Yes i'am talking about the .257 stw.
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Unread 09-12-2004, 10:34 AM
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Join Date: Jun 2004
Location: Fort Shaw, Montana
Posts: 6,848
Re: Neck sizing


If you can slide a bullet in teh mouth of a fired case you have no need to turn your necks as far as a safety matter goes.

One easy way to check to make sure you have enough neck to chamber clearance to be safe is to take one of your fired cases and measure the neck diameter with a quality micrometer, a quality set of calipers can work but is no where near as accurate.

Then measure the neck diameter of one of your loaded rounds. If you neck size, make sure your measurement is on teh sized section of the neck.

Take this measurement and subtract it from the first cases measurement and you whould have at least 0.0015" differnce with the fired case being the largest.

With a 0.0015" measrement, you have 0.00075" clearance on either side of the bullet when you fire a round. THis is the absolute minimum you should have for safety.

I will walk you through this with my 257 STW cases.

My fired cases with top loads have a neck diameter of 0.2902".

My loaded rounds using a 100 gr Partition or Ballistic Tip measure 0.2882".

Subtracting the loaded diameter from the fired case diameter leaves, 0.002".

This measurement divided by half (because the total measurement is the result of the clearence on both sides combined) give you the clearance arounf the bullet when the case is fired.

In this case, there is 0.001" clearance which I feel is about perfect. You can go as tight as 0.00075" clearance and I do in my +1000 yard wildcats that I fit the neck to the chamber but for 95% of all shooting, 0.001" clearance is perfect as far as safety, allowing the most velocity without increasing pressure and accuracy.

Factory chambers will be in the 0.005" to even as loose as 0.015" when you measure a fired case compared to the loaded ammo. This is a major resaon why they do not shoot anywhere near as consistantly as a custom chamber.

IF your cases do not allow a bullet to slide easily in the mouth, you probably need to turn your necks until you get that 0.001" clearance for safety.

Also, make sure you have your pressure up, very low pressures will not open the case mouth up and you will get inaccurate measurements.

Good Shooting!!

Kirby Allen(50)
Kirby Allen(50)

Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.

Farther, Faster and Flatter then ever before.

Web Page: www.apsrifles.com

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Unread 09-12-2004, 11:47 PM
Bronze Member
Join Date: Sep 2004
Location: SC
Posts: 48
Re: Neck sizing

Thanks kirby!
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