The Science of Annealing — Facts Uncovered, Myths Busted
The science behind annealing during the manufacture of new cases is well-established. What happens after that, when we repeatedly reload and anneal those same cases, has always been somewhat of a “dark art”. To help separate scientific fact from fiction, the creators of the Annealing Made Perfect (AMP) Annealer machine have conducted detailed studies of cartridge brass. The AMP Team’s studies offer some remarkable insights, while disproving a number of myths about annealing. Will annealing tighten your groups? The evidence of these studies shows it could.
The test results are fascinating. The team compared brands of brass, sectioning brass to examine both alloy composition and thickness from case mouth to case-head (bottom). They also examined how carbon build-up affects next tension. And they determined how brass changes over multiple loading cycles. They even did a series of bullet-pull tests to analyze factors affecting neck tension. Here are some of the key subjects in the reports:
Brand by Brand Analysis — How the cartridge brass alloy varies among different manufacturers.
Bullet Release and Neck Tension — Tensile Bullet-Pull tests show factors affecting neck tension.
Neck Tension and Carbon — How carbon build-up inside the neck affects “neck tension”.
SS Tumbling and Hardness – How tumbling with stainless media affects brass hardness.
Case Cleaning (Ultrasound and Tumbling) — How case cleaning affects annealing.
Multiple Loadings — How brass performs when annealed every reload over 10+ cycles.
You really should read the reports — there are some fascinating revelations. The AMP team made longitudinal sections of various cases to show different case wall thicknesses and head geometry. These examples also show how the hardness of the case varies from the case mouth to the case-head. Both virgin and used, annealed cases were examined.
Bullet-Pull Tests — Using advanced tensile test equipment, AMP experimented with different combinations of dies, reloading sequences, and neck hardness to ascertain the best practice.
Carbon Inside Your Case-Necks May Be a GOOD Thing
AMP’s testers found carbon in necks can be beneficial: “Even with identical interference fit and neck hardness, as the carbon layer increased (microscopically), the force to draw the bullet decreased. It would appear the carbon acted as a lubricant. Interestingly, the [pull force] standard deviation also improved, i.e. the case to case variation in the force required to draw the bullets decreased.”*
Read the Full Test Reports
The AMP team’s objectives were to clarify some misconceptions on just what annealing does and does not do, and also to establish the best practices for consistent results. They have consulted with three independent certified metallurgy laboratories to produce some definitive information. So far, the Stage 1 and Stage 2 reports have been released. The studies include a report on the general physical properties of cartridge brass, including grain structures, hardness scales, time/temperature annealing information, and what can cause de-zincification.
The FULL REPORTS, including comprehensive appendices, are found here:
Stage One: https://www.ampannealing.com/articles/40/annealing-under-the-microscope/
Stage Two: https://www.ampannealing.com/articles/42/annealing-under-the-microscope/
Examining Different Brands of Brass — What the Tests Revealed
Is Lapua brass harder than Norma? Is Lake City better than Remington? You’ll find answers to these and other questions in AMP’s annealing studies. One of the key findings in Stage 2 of Amp’s research is that brass from different manufacturers does vary in the distribution of material in the walls of the case.
Stage Two Conclusions:
— Different brands of the same cartridge cases can require different annealing power settings due to differing case wall thickness in the neck and shoulder region. The greater the mass of brass to be annealed, the greater the power requirement. Lot to lot variation within the same brand can occur for the same reason.
— The bushing die used in this set of tensile bullet pull tests gave significantly more consistent results than the standard neck die with expander ball.
— Cases should be annealed every reload in order to get the best repeatability.
Case Variations: Brand to Brand, and Lot to Lot
Here is a sample from AMP’s test report:
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Analyzing Different Brands of Brass Both the Lapua and Norma neck walls are 314* microns (0.01236”) at the mouth. The Lapua neck wall thickens to 348 microns at the junction of the neck and shoulder, and the Norma neck thickens to 325 microns. Through the shoulder, however, the walls of both cases thicken to 370 – 380 microns. Once past the shoulder, they both taper back to 314 microns, before starting to thicken again, moving towards the case head. The Lapua case requires AMP Program 47 to anneal correctly. It is the heaviest of the four cases tested through the shoulder region. The Norma case, which is only slightly lighter through the same region, needs Program 43. The Remington case is very similar to the Lapua and Norma cases in the neck region, but it actually thins fractionally through the shoulder and front section of the body. The AMP program setting for Remington 223R is P32. The Lake City case is the thinnest throughout of all four samples. It only requires Program 28. The above samples clearly demonstrate that the mass of brass to be annealed is critical to the power requirement for correct annealing. |
To see how the AMP Induction Annealing Machine works, watch this video:
* However, in Stage Two of AMP testing, the testers experimented with clean, carbon-free necks with dry lube. There was some indication of greater tensile pull consistency with dry-lube, but AMP plans to do more testing.
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Tags: AMP, Annealing, Annealing Made Perfect, Lake City, Lapua, Neck Tension, Neck-Turning, New Zealand, Norma, Remington
This is an extremely good read, well worth the time of any serious reloader. While I am still in the process of reading it, I thought that I would share one quote, that affirms what many have said over the years.”…even with identical interference fit and neck hardness, as the carbon layer increased (microscopically), the force to draw the bullet decreased. It would appear the carbon acted as a lubricant. Interestingly, the standard deviation also improved i.e. the case to case variation in the force required to draw the bullets decreased.”
Keep in mind that ‘pull force’ itself is not neck tension. Tension can only be relatively represented by pull force, while friction is held at a constant.
I’m all for the carbon layer, it’s all good. But taking necks to squeaky clean does not affect tension. It only affects seating/pull force. Actual tension, the spring back gripping force on seated bullets, is independent of friction.
And the “drag” on the bullet inside the neck, lasts about as long as it takes for 20 tons/sq. inch to blow the case neck out hard against the chamber walls whilst simultaneously driving the (much heavier) bullet into the throat.
Having the bullet hard up against the lands at launch, will, as pretty much everyone has noticed, produce a different pressure curve and mean velocity than if a little free-bore exists.
What has been interesting is the apparent change in the general “tightening” of groups when the Lee “factory crimp die is employed.
Purists will blanch at the thought of “mutilating” the case mouth, but, for those shooting “service” or actual “field” rifles, the elimination of “bullet shuffle” in the magazine is not to be sneezed at.
It comes at the slight cost of reduced neck life, but, in a game-getting rifle, this is of little importance.
If your piggy-bank runs to belt-fed toys, serious bullet “retention” is vital.
I’d sure be curious to know what the ratio is between the force needed to push a bulket out of the neck, versus through a long, rifled barrel while engraving it. I’d tend to think that the spread in neck tension in a 100 count batch of brass is a very, very tiny fraction of that needed force, perhaps on an order of magnitude of less than one kernel of powder in a rifle case.
Excellent article Alex. Thank you for your time in researching and compiling the data, and publishing it for free! This is such valuable information and gives us all a much deeper understanding of what really happens to our brass. You’re a scholar and a gentleman. Huge appreciation and best wishes.
I’l back up Bruce’s observation about crimping for service rifle or field use. If one isn’t annealing brass every time, adding a factory crimp die into the reloading sequence seems to be a viable option to keep round to round consistency for all brass with the same number of reloadings. It’s a good option for short line service rifle ammo where your brass count per prep cycle numbers in the 1,000 to 2,000 pieces at a time. The thinking here is a bit different because SD’s matter much less than shot technique at that end of the game.
For applications where super repeatability in take offf speed are critical, I’d break out the annealer. This article and the referenced sources are quite useful here in that they show some science based reasonableness in an area too often shrouded in myths. Keeping brass isolated by number of firings means quite a lot as it turns out.
carbon layer in neck region…
it is good for case to case consistency, but very bad for batch to batch consistency.
which one to choose for our goal for reloading?
As you mentioned, it was the annealing of the brass that made it stronger in the long run. We are thinking about putting a metal raining around our deck in the backyard. Do we need to make sure that it is annealed first? Does that process change the look of the metal?