This one’s for Don and the other folks out there scratching their heads about neck tension and how it applies to reloading. To put it simply, neck tension is how tight the neck is holding the bullet from slipping forward into the rifling as pressure builds in the case when fired. Instead of being measured with physics terms like friction and force, it’s typically referred to in thousandths of an inch. How does all that apply to your hand loading practices, though? Does it matter at all? Should you be annealing? We’re going to cover all of it in this article on the merits of neck tension!
I’m going to say this from the get go, do what works for you! In my experience, neck tension is one of the things that you should be paying attention to when you hand load your own ammunition. The reason is, I’ve seen it’s negative effects. The biggest example or story I can think of has to do with sizing 6.5x47L brass down to 6x47L. The first few batches I necked down I used a bushing die to do it. Bushing reloading dies are useful because you can select different size bushings to really control the neck tension. My problem is, it wasn’t the best tool for this task. The bushing doesn’t size the entire neck, so in sizing down the necks, I was getting inconsistent neck tension. Part of the neck would be at the 6.5mm size and part of it was at the 6mm size after I ran the neck through the die.
How did I know neck tension was an issue? The easiest things you’ll see to determine inconsistent neck tension are how much force it takes to seat your bullet as you lower the handle on your press and the consistency of seating depth. If you seat ten bullets into cases and they all have different seating depths, that’s probably inconsistent neck tension. The bullet will bind and grab the bullet on tighter cases so the bullets will seat longer. The reverse is also true, on a loose case the bullet will push into the case with less effort, though it likely won’t affect the depth the way a tighter neck will.
Neck Tension Nuance
I think neck tension is probably one of the keys to producing single digit SD ammunition at home. I didn’t throw those bushing dies away for my 6x47L, I just use a full length die for necking down now. On all subsequent firings of the brass I will typically size the brass with a Redding Type-S full length sizing die with a bushing halfway between the fired diameter, and the resized diameter. After tumbling, cleaning, etc. I will run the brass through a Redding Type-S neck die with the final size bushing that I want the neck sized too. This is the second to last operation in my reloading process. The only thing that comes after this, before powder and bullets, is trimming.
When discussing neck tension, it’s typically described in thousandths of an inch. So say you measure the outside neck diameter of fired 308 brass and you get a measurement of 0.340.” A common neck tension settings is two thousandths, so 0.002″ worth of tension. Say after sizing the brass it drops down to 0.330″ for the outer diameter. When you seat a bullet, the neck will swell and expand around the bullet. Say the diameter of the neck then measures 0.332,” well then you’ve arrived at two thousandths of neck tension. The easiest way to do all this is load a round with your components, measure the neck diameter, and then subtract those two thousandths. The outer diameter, minus the desired tension in thousandths, is the size of the bushing you want for sizing the neck.
Why size the neck in two steps? One of the things that can affect neck tension, or is thought to, is the work hardening of brass. As it’s used it becomes more brittle and springs back farther than when it’s new. So picture squeezing the neck down inside the bushing to set the tension, but now it springs back out more than it did initially. That would lead to a looser neck over time. The theory is if you make a big jump in neck diameter in one shot, it’s more work hardening than if you do it in two steps. Honestly, I’m just a bit anal about neck tension and I think it really matters so that’s why my second to last operation for brass preparation is running the necks through a bushing neck die. I want to make absolutely sure the neck is sized the way I want and consistent before I start seating bullets.
Other Neck Tension Stuffs
You may have heard of annealing. This is the practice of heating the necks of the brass, uniformly, and to a consistent temperature. This then softens the brass and, in theory, enhances the longevity of the brass and leads to more consistent neck tension. I’ll just toss this out there right now; the Applied Ballistics guys have done a lot of testing on annealing and the results are in Modern Advancements in Long Range Shooting Volume II. I’m not going to spoil that for you except to say it’s probably not worth doing until you’ve got in excess of ten firings on the cases. Annealing every 3rd, 6th firing, etc. is unlikely to result in any difference in neck tension or accuracy. If you do have double digit firings on brass and you plan to use it further, there are folks out there who will clean and anneal the cases for you at around $0.10/case.
Loose necks can be detrimental to accuracy. Again, that’s a conclusion I’ve seen in my own reloading over the years. I played with it a bit and found that accuracy was better with two to three thousandths of neck tension than with less. What I noticed was a velocity increase as the pressure had to build a bit more, I guess, to get the projectile moving. The end result was likely a higher, and more consistent, muzzle velocity which resulted in better accuracy when I was shooting my 308 in the first months of hand loading.
There are many myths and legends when it comes to hand loading. Some stuff matters and some stuff doesn’t. I think different things matter for different disciplines of shooting. If you shoot Benchrest and the top 10 shooters are separated by tiny fractions of an inch in group size, things like sorting your brass by weight makes sense. Sorting bullets by weight along with trimming meplats and tipping bullets can have their place. However, we cater to tactical style shooting on this site where the targets are bigger but the shooting positions and setup before the shot are way worse. As a result I try to give you guys the best advice. I don’t think weight sorting brass is worth your time for tactical shooting. I do think a bushing die and some bushings are worth the money to ensure consistent neck tension on your loaded rounds. If you have any experience you want to share on the subject, do so in the comments below!