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Measuring Case Neck Thickness

Q

Quackaddict

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Minuteman
Nov 7, 2009
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Minnesota
#1
I am trying to figure out the bushing size for a set of 6.5 PRC dies. Using a standard micrometer (I realize this is not the ideal tool for this) I am getting neck thickness of .11 on ADG brass when I measure the thickness of one side. If I take an inside/ outside measurement, subtract, and divide by two I get a thickness of .014.

When I add (2x.11) to .264 I get .286, minus .002 I get a bushing size of .284. The trouble I am having is that this number is no where near anything else as far as I can see. I have some loaded hornaday 147s that measure .290, and a user on facebook posted loaded ADG brass on a 6.5 SAUM needing a .289 bushing. If I use the .14 I get a .290 for a total diameter an a .289 bushing seems to be right.

So the million dollar question is what number do I trust? Do I go off the inside/outside measurement? I'm not opposed to buying a couple bushings but I need to load ammunition to get the "perfect" measurement first.
 
#6
1549917803013.png
 
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#9
Is this what you mean by standard micrometer? If so then I would trust the outside measurements taken with it.
(outside diameter of the loaded round-diameter of a bullet) / 2=neck thickness
1549918356599.png


Those style ^ are not a great fit for measuring only one side's thickness, a ball or tube micrometer would be better so that it would fit the inside curve of the neck.


Ball micrometer-
1549918534415.png


Tube micrometer
1549918652919.png
 
#13
Between a ball caliper and the case neck gage, which one would be most useful long term? If I have unsized brass will tbe case neck gage still be accurate?

I have no sizing or seating die to make a loaded round, hence needing to measure.
 
#14
Quackaddict said:
Between a ball caliper and the case neck gage, which one would be most useful long term? If I have unsized brass will tbe case neck gage still be accurate?

I have no sizing or seating die to make a loaded round, hence needing to measure.
Click to expand...

I would get a ball micrometer similar to this. I have Sinclair's digital gauge but it doesnt look like they sell them anymore.
 
#15
Not trying to rain on anyone's parade, but I'm not a fan of digital calipers or mics. Things may have changed, but last I looked, digitals could only resolve to a half a thousandth (.0005"). Whereas, with analogs....you have a chance to fairly accurately "resolve" (via eyeballing it) to a third, maybe a quarter of a thou (.00033" to .00025"). The Mitutoyo Tubing mic that I have (shown in multiple posts above) is just the shizzle. The feel of it is great and the repeatability of it is fantastic. I bought mine about 20 years ago and I think I paid $80 or $85 for it. If I had to replace it today at twice that price, I'd gladly do so.
 
#16
Quackaddict said:
Between a ball caliper and the case neck gage, which one would be most useful long term? If I have unsized brass will tbe case neck gage still be accurate?

I have no sizing or seating die to make a loaded round, hence needing to measure.
Click to expand...

Youre using your terms incorrectly making what you say unclear. No such thing as a ball caliper.

Caliper- good for .001"
1549925131587.png


Micrometer- good for .0001"
1549925164912.png



For what we do a caliper is probably the easiest, quickest and most adaptable. So take your lyman caliper that you already have and measure the outside diameter of a loaded round. Choose your bushing size off that measurement.
1549925385895.png


Take that measurement in 5 locations around the case neck and over five sample cases. Find the most frequent number and use that.

You said that you did that and then broke it down and got to .014"
So that means your outside loaded diameter was .292" then, correct?


If so: are you intending to run an expander ball inside of your full length die? If so choose a bushing .004" smaller than the loaded diameter (woud be .288), the ball will open it up to .002 under loaded diameter (.290).


If you are not going to run an expander then choose a bushing .002 under (.290).
 

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#18
I'm sorry for the term confusion in my posts. Let me see if I can clarify.

When I measure a single neck wall with a caliper I get .011. When I measure an unloaded un sized piece of brass inside to inside and outside to outside (with said calipers) I get a neck thickness of .014. This variation is the reason for the post. I just don't want to end up with a bushing that is way to small or not small enough.

I don't have a die set to make a loaded cartridge. I was going to call Redding today and order a set, but got caught up with the bushing size. Hence I cannot just simply load a few a find out what a loaded cartridge measures and select a bushing from there.

I ordered a ball micrometer to get some accurate measurements of neck thickness. I figured there was the right tool somewhere I just didn't know what was right.

Spife, why does neck tension change with removal of the expander ball? What is the benefit of removing it? I will be doing some reading tonight, jus interested to hear what you have to say.
 
#19
When you size brass down with the bushing it will push any neck thickness irregularities towards the inside leaving the outside perfectly round at the cost of your inside which is where the bullet interacts with the brass.

If you expand you then push those irregularities to the outside where they are floating in space and it leaves the inside perfectly round for a nice interface with the bullet.

If you choose a bushing that leaves the brass the same size as the expander ball then it doesn’t do anything really. Brass is elastic and those thicker parts that the ball might just barely skim over will spring back to where they were. For the ball to work it really has to move in an appreciable amount hence the sizing it down with a bushing .004 low so that the brass can then open it up to the proper .002 under (working the neck open .001 per side).

If you’re using good brass or neck turning you won’t really have any irregularities so you would be fine with just the bushing. Cheaper brass necks vary much more in thickness than those good or turned cases do so it makes a more appreciable difference in the final neck tension/interference fit/bullet grip to expand the insides to the same.

(Or so the theory goes)
 
#20
Based on what you are trying to do, the easiest and quickest thing to do is measure the diameter of a few loaded rounds using your caliper. Measure at the center of the neck, and take 4 readings (1/4 turn around the neck each time). Get the average of your measurements and then subtract the amount for the amount of neck tension you want. 100 yards 0.002 is a good starting point, 300+ 0.003 is a good starting point. Using your original example, if your loaded round was .270, you want a .268 (-.002) or .267 bushing (-.003).

So basically you were headed in the right direction the first time just applied it wrong. Don't worry about the neck thickness, just focus on the loaded diameter.
 
#21
Spife, its starting to make sense....

With your example, you need to size down more WITH an expander to allow enough interface to let the expander work. This would be useful with shitty brass or brass with poor neck consistency so the inside is more round that the outside.

WITHOUT an expander, you are only sizing the brass down to accept a bullet. The advantage is less working of the brass which should lead to onger case life. This approach is best used with quality or neck turned brass.

If you were going to use the non expander method I would assume you would use the expander any time you brought in new brass or had mouths that were dented in correct?

Thank you all for the information and the time to explain what is going on with this...
 
#22
Yep, you can run it into the die just enough for it to round out the necks and the expander to catch. I wouldn’t go completely full length sizing new brass though which is already on the small side of the spec so just deep enough for the necks.
They also make a stand alone die (expander die body) that utilizes expanding mandrels (aka turning arbors) for that same reason, expand the necks even more uniformly and fix dents etc.
 
#23
An update to this thread, with handy ball micrometer in hand, I was able to make solid measurements of the case necks. Across 5 cases and across several points on each case I have an average of .015 thickness. The Hornaday case I have runs about .014.

.264 + .030 = .294, which means a bushing size of .291 or .292 would be money.
 
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