Neck Tension

[7mm Eclipse]

I've read recommendations for using the neck tension that is generated when using .001, or .002 to .003, or a minimum of .003 reduction in the loaded round neck OD.

What are the pros and cons on having higher or lower bullet holding tension in the neck?

Is there one recommendation for tension levels used to hold the bullet in a clip loaded round and a different recommendation for rounds that are too long for clips and that have to be loadedby hand one at a time into the chamber?

Thanks for sharing your thoughts and experience.

 

[boomtube]

Okay, a good question but I hate to write long posts.

There really isn't any grip difference in "bullet tension" after necks are about 1 thou under size, the rest is just permanetly stretched during seating. The increased resistance we feel when seating in smaller necks is only the added effort required to expand the neck with the bullet, it's not real "bullet tension/grip" at all and that increased seating effort does have a "con" I'll touch on below. And don't take my word for this, you need to do an experiment to prove what I say to yourself.

Size two cases, expanding one neck as you normally do, then remove the expander plug and size the other without expanding. Mike the outside of each loaded neck about mid point, marking the location of mike contact so you can return to the same point later. Record the numbers.

Seat a bullet in each so shank contact with the neck runs to the shoulder (using a boat tail will make seating easier in the smaller neck). Notice the effort required to seat in both cases, it should be easy to tell which one was normally expanded and that incorrectly suggests it's gripped tighter. Mike the necks again, same point as before, and write that diameter on your note sheet.

Pull the bullets, measure and record the neck diameters a third time.

No matter how much smaller your unexpanded neck was before seating, both will be virtually the same, within the limits of your ability to measure them, and they will only be about 1 thou smaller than the loaded diameter. With this you will see and have proven there is no significant grip difference in a sized fit smaller than the final difference you measure, the rest simply stretches the brass past its elastic limits.

Smaller necks - "high tension" - seating harms accuracy by producing increased bullet run-out. Driving a bullet into a too small neck forces it to be the final expander and that increased resistance tends to tilt the bullets as they enter, making them enter off-axis and increases run-out for absolutely no benefit in grip.

Bottom line, there is no advantage to making neck inside diameters smaller than 1 to 1.5 thou and there are some very real disadvantages to going smaller than that.

 

[Mikecr]

Perfect Boomtube!

i'll add;
What can be done to change tension is stress relieving('called' annealing) for less, or adjusting the length of neck sizing more or less.
Magazine fed guns that recoil alot, or feed fast, could benefit from higher tension to hold preferred seating depths.
I don't have these guns, and I partial NS one cal of neck length as a starting point(down 1thou after springback).
I can adjust this length, and do, during load development.

 

[boomtube]

"I don't have these guns, and I partial NS one cal of neck length as a starting point(down 1thou after springback)."

You make some good points.

I'll add that there is no benefit to neck sizing below the base of a flat bottom bullet or below the taper of a boat tail.

 

[SHRTSHTR]

Another good thread here guys.

I just recieved my Neil Jones dies for a 338 LAI. I noticed the neck sizing bushings it came with are .362-.363 diameter. My neck turned Lapua brass with a loaded round measures .366 I would prefer to have .0015-.002 neck tension. I think that using the .363 bushing would be .001 to much. Would like to hear what you guys think?

I do have a way around this. I ordered extra sleeves for my Redding competition dies and reamed out the sleeves on my Toolroom lathe with my PT&G reamer. I have the appropriate bushings to get whatever neck tension I want.

Depending on what I hear hear, I would like a little larger bushings for my NJ dies. I have a little money invested in these dies not to use them.

 

[woods]

I agree that you reach maximum usable bullet grip at a certain point. In my tests like boomtube described above the neck ID returned to .002" less than caliber. These were measurements taken on the ID with pin gauges.

Personally I size the neck to .003" less than caliber and when looking for increased runout I haven't encountered any. I also use mica or Imperial dry neck lube and throughly prep the inside of the necks to ease seating. Since IMO .002" is the number where additional stretching is useless, I prefer to start at .003" in order to try for consistant stretching.

YMMV

 

[7mm Eclipse]

Thanks for all of the responses. A lot of really good information.

I will run the recommended test checking the neck sizing with and without the expander then measuring the neck diameters of both as sized, with bullets and with bullets pulled to find the amount of spring back for my Norma Brass.

The point about necking to smaller ID's effecting the run out is appreciated.

I was also a little concerned about the smaller ID sizing causing the brass in the neck to "work harden" faster with all of the additional streching resulting in possible variations in sizing and in brass life.

Since I load into or close to the lands with my rifles, I don't use my clips. It looks like for my specific applications the .001 to .0015 range may be the ticket. I will wait and see what the above spring back test data shows.

Thanks again for your time and assistance.