Home Forums Hunting & Shooting Ask The Gunwriters How to determine neck thickness

Alright guys, say I have my parent case, but it's going to be necked up wildly.
How do I figure the thickness of the brass at neck?
I understand that fire-forming is THE way to neck up, but I can't fire-form w/o reamer, can't have reamer w/o dimiensions.
Seems I can just get and expander to open the parent case up to see, but it'll be thicker pushed down than blown out.
If I guess, I'll have to guess on the inside, and turn all my cases, right?
Is there a known formula for the movement of brass?
Please help!

To make it even more complicated, the neck will be what was the shoulder, so I'll be reaming the bullet diameter.

Not sure how to get there from here!
????

As you form it up (larger in diameter), the brass gets thinner (stretch a wide rubber band and watch it get narrower). If you form the old shoulder out very far, it may become thinner than the original neck.

To calculate the change, first subtract the area of the inner circle (inside diameter) from the area of the outer circle (outside diameter) of the original shoulder at the point where the new neck will begin. The difference will be the cross-sectional area of the brass at that point. Then calculate the area of the larger inner circle (after necking-out) and add the area of the old cross-section to see how large the new outside diameter will be. There you'll have, at least theoretically (in the difference between the two new diameters), the wall thickness to expect after you've formed the larger neck.

Let
A = area,
D = diameter, and
R = radius
in each calculation of the area of a circle.

In each such calculation, A = 3.141592654 � R � R
and of course R = � � D

Good Lord, I'm glad I didn't have to figure that out!!
Thanks Doc!

This is an interesting question.

When you subtract the area of the outer circle from that of the inner circle as described, you get the area of the brass. Easy enough.
When you change the caliber, brass becomes thinner or thicker as the amount of material obviously stays the same. Easy enough.

The difficult part is:
a) chances are fair that wall thickness will not change proportionally, i.e. checking the wall thickness of the neck is imperative and mostly you need to turn it
b) the cristalline structure of brass is changed (drastically- when you go from .221 to .300 Whisper, for example) which means you have to anneal the necks after forming. If you skip this step, you may find the neck cracked when seating the bullet or after firing the first round.

The unanswered question is: what is the minimum wall thickness (mostly answered by try and error), and (academic): how much stress is applied to the brass (academic) ?

"When you subtract the area of the outer circle from that of the inner circle as described, you get the area of the brass."

Vice versa works better. Gives the same number, but positive, not negative -- somewhat more logical and realistic. (This comment intended for the benefit of the readers, not the correction of the writer, of the line quoted above.)

You are right - noticed it but was too lazy to change it in spite of its lack of logic and reality :-).

I wonder how a piece of negative material looks like - never seen any yet.


But what's about minimum brass thickness and stress ?