Re: Measuring case neck tension
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Fuzzball</div><div class="ubbcode-body">"Most people tend to agree that the maximum "neck tension" you can achieve is what you get when your bullet is seated in a neck that measures .003" less than bullet dia."
Yeah, that's a popular idea and intutively it seems it should be that way. But if you carefully measure a loaded cartridge neck, pull the bullet and measure again at the same place you'll fine the difference to only be about 1 thou so that's all the springback 'tension' you're going to get no matter how much smaller you size the necks. </div></div>
Well, of course. The bullet is just going to work like an expander ball, pushing the neck out to the bullet diameter, and then it will spring back to .001".
We express neck tension in thousandths of an inch diameter difference just because most hand loaders lack the tools to measure it any other way. The true way to do it is to measure seating force. K&M has an arbor press with an attachment for a gauge to measure seating force. I don't use an arbor press for seating, but if I did, I would use the K&M press with the gauge.
Most people who don't have the K&M press seat by feel. You can usually tell if a bullet is off (meaning harder to seat). Truthfully, there are a lot of different variables that go into how tight that neck is holding on to the bullet. For instance, how much of the bearing surface of the bullet is contacting the neck wall, amount of friction between neck wall and bullet (cleaning meathod can come into play here, in US cleaning, you can get the neck so clean, the friction is higher), whether the brass has been annealed (unannealed necks have less spring back, therefore more tension, expecially if the brass has been sized a number of times without annealing), even variances in bullet diameter.
I recently loaded up a bunch of 185 Berger Hybrids and noticed there was a lot more seating force than when I seat 175 SMK's in the same brass. I went up a bushing diameter to reduce the tension, but now it is a little too light. I was thinking about calling Neil Jones and seeing if he made bushings in half a thou increments.
So back to the answer to the question, seating force is the ultimate metric, and absent a tool, you will need to know seating force by feel. This only comes with experience.
Here is an old trick to see if you have not enough neck tension... push the bullet tip of a load cartridge against a block of wood. If you can seat the bullet farther just grasping the cartridge brass and pushing down, your neck tension is too light for practical applications.
