Well, strictly speaking, you can, in fact, calculate tensile stress around the neck from a combination of the change in cross-sectional area of the neck due to the measured expansion of the neck OD over the bullet and the change in neck wall thickness it causes using the modulus of elasticity (16,000,000 psi for 70:30 cartridge brass) and Poisson's ratio (0.375 for 70:30 brass). Once you have that, you can work backward through the hoop stress calculation to find the pressure applied inward against the neck. When you multiply that times the contact area between the neck and bullet and by the static coefficient of friction between gilding metal and cartridge brass, the area will have canceled out and you would have bullet pull in pounds.
However, that is all in theory and not of much practical value because of all the places it can go wrong. Your case alloy probably isn't exactly 70:30 brass. Your bullet maker may use a slightly different gilding metal alloy than average. Your coefficient of friction probably won't match perfectly from case-to-case due to neck lube or the presence of carbon or how smooth the surface is. SS pin cleaning might fix the coefficient of friction variation, but even so, that number would only be good for a while, as brass and gilding metal tend to begin to stick together over time.
So, bottom line, yes, tension in psi can be calculated and bullet pull calculated from the result, but why the dickens would you bother when measuring bullet pull directly would be so much easier and gets around the alloy and other variables?
cw308 said:
What I'm getting from your post , you run a case up into the die , raise the stem and expander untill you feel contact with the bottom of the case neck . Correct? ...
That will be too snug. The brass can't really turn the corner as quickly as that would demand, so it would just jam on the upstroke. The idea is just to have the bottom of the neck engage the expander before the case mouth leaves the die. This is to get the neck started straight over the expander and without sideways pull. The position of the expander can be determined by turning the stem up until it contacts the brass at the neck/shoulder junction, as you described, and then turning it back down from that point by a distance equal to about 2/3 to 3/4 of the length of the case neck. The proportions of necks are so variable I can't give you an exact amount. With some of the WSM cases, it might not work at all. But basically, the closer you can get the expander to the neck portion of the die without feeling it start to increase your resizing or withdrawing effort, the better alignment with the expander should be imposed by it.
I can see situations in which the above may not work well due to poor die and press alignment or uneven drag on the expander due to uneven lubricity or neck wall thickness pushing the expander to the side and letting it shift alignment after the neck mouth clears the die. You'll have to experiment a bit to get the most out of it.
Hounddawg,
If he pushed the die until it popped the aluminum plug, I think the problem he had was the mandrel being a little wide. Lee makes them only about 0.001"-0.002" under bullet OD, and if you get one on the larger size of the tolerance range or a bullet on the narrower side, and especially if your brass needs annealing to take the spring out of the neck, you can end up needing a narrower mandrel than the die came with. A number of users report having to reduce them 0.001".
