Some of us know that, of the 3 components of the Holy Trinity of precision ammo (powder, seating depth and neck tension). Neck tension is the most difficult to control.
I know that everyone likes to control neck tension with bushings and an expander ball, but to me that doesn't seem like a very good way. From my way of thinking bushings transfer any variance in neck thickness to the inside of the neck where they have a direct impact on neck tension. Then when you pull the expander ball back out all kinds of nasty little things happen. That is not a good thing, because no matter how careful you are turning necks there will be variances, and those variances will impact neck tension.
I believe that the outside of the neck has minimal importance, as long as there is a minimum .003 neck clearance for bullet release. The only important thing is how thick the necks are because that has an influence on spring-back. IMO the part of the neck everyone should be focusing on is the inside of the neck, or more appropriately inside diameter. The inside diameter is what influences neck tension, if one can transfer neck wall variances to the outside of the case it is that much better.
So the question is:
How do we control neck tension from the inside of the neck? What if instead of using bushings we were to use a mandrel to expand the whole neck at once. With bushings one has to be twice as accurate then with a mandrel. What if we were to use a mandrel to expand the necks to provide the desired neck tension? What would a mandrel do?
Well, ... a mandrel would control the inside diameter of a case neck much more precisely than any bushing ever can. It could also transfer neck thickness variances to the outside of the neck where it doesn't interfere with neck tension and bullet release. That's good isn't it?
I have been using mandrels for thousands of cases along with a Hydro press. The problem is that just when I have a mandrel that gives ideal neck tension (down to .0001 precision) it starts to wear, and after a couple of thousand cases it is done. Making another mandrel to .0001 precision is very difficult/not possible for most machine shops and grinding shops. So I set out to look for a solution. I eventually focused on the Hornady bullet puller collet, because that's what I had. I started using the bullet puller to hold pin gages to expand the necks to match the turner mandrel.
However, the bullet puller did not hold the pin gages as co-axially straight as I wanted, so I approached Kenny Porter, who is tool and die maker for all kinds of things including parts for pacemakers (can you think of anything more critical?). I told him I wanted him to make a die for me that would hold a pin gage, actually a range of pin gages ie .305, .306, .307, .308 and .309. Once I showed him what I had in mind he made a leap to a collet that is ideally suited for what I wanted. The collet holds the cutting bit for a CNC machine. It has real holding power.
Pin gages are small tubes 2" long and can be bought for a few dollars and they are available in dimensions as exact as .0001 (+/- .00002). They generally have a 60 to 62 Rockwell hardness, like tool steel. They are ideal for this purpose, all you have to do is bevel the tip.
Now, when I need a new mandrel I buy a Vermont Gage pin gauge from Amazon, chuck it in my drill and move it against a grinding stone until I get the desired point. If you’re looking for +/- .0002 precision they are round $3 each. If you really want to get precise and have a mandrel to .0001 (+/- .00002) precision then the price jumps to around $18 and you’ll have to order directly from Vermont Gage. For $18 you get a mandrel and a certificate of accuracy and the name of the guy that inspected and when.
Here’ the mandrel die that Kenny Porter (
kennethp@portersprecisionproducts.com ) made for me.
View attachment 1078330
View attachment 1078331
View attachment 1078332
This mandrel die makes all other mandrel die I have seen or bought look primitive.
Regards
Joe
PS This is a cut-n-paste from a previous post of mine.
