The term ‘annealing’ might not be familiar to everyone, especially those accustomed to factory-made ammunition. However, its significance cannot be overstated. Our resident expert, Paul Bradley – Ballistician, Hexagon Ammunition (Beretta Group) – uncovers how annealing contributes to the durability and reliability of ammunition, ensuring both safety and consistency in performance.
The process and purpose of annealing
Reloaders may well be aware of annealing but many people who purchase factory-made ammunition are largely unaware of it. You may have noticed the neck of your brass cases have some rather pretty discoloration, often slightly blue or orange. This is a sign that annealing has occurred but what exactly is it and why do we do it?
Cases are formed from a brass cup. The forming process stretches and bends the brass into the required shape. Before the brass is worked the internal structure of the metal could be described as a lattice free of defects. As it is bent and stretched the lattice becomes dislocated. The lattice becomes more resistant to further dislocations occurring and this manifests as resistance to deformation. In short, the material becomes tougher, but less malleable. This is known as “work hardening” which can be desirable in some contexts and not so much in others.
The formation of the case is done in multiple stages. This is because the brass work hardens as it is drawn. An annealing process takes place after some of these stages. Annealing is the process of heating up the metal which causes changes to the lattice structure previously mentioned. The dislocations in the lattice are removed by heat and the metal returns to its more malleable original state. Work hardening is somewhat of a negative aspect at this point as it adds process (and thus cost). If no work hardening occurred, the cups could simply be formed by multiple dies in a single machine and a completed case would drop off the conveyor belt.
Once the case has been formed it is annealed at the neck. The neck of the case is now softer than the head section (the base is called “the head”).
The role of annealing in performance and safety
A primer is inserted into the primer pocket and is held in place with an interference fit. In simple terms it takes force to push it into the pocket and thus is held firmly in place once inserted. The case is then filled with propellant powder. A projectile (bullet) is inserted into the neck. Because the neck has been annealed it is softer and as the projectile is pushed in the neck springs back to grip it at a consistent level of force. This force can be measured with an extraction test to ensure the projectiles are held firmly but not so much that a dangerous peak in pressure would occur when hot gases push the bullet out of the neck and down the bore. The important factor, apart from the safety aspect, is consistency of neck tension. Consistent neck tension will allow consistent pressure and thus consistent velocity.
Why is consistent velocity so important? Because it will play a major factor in the accuracy and precision of the system. At extended ranges a significant drop in velocity will cause the projectile to strike low or short of the target. An increase in velocity will produce misses over the top. Neck tension is one of several variables that effect velocity consistency.
Hot gases create a great deal of pressure within the case which expands (fireforms) to the inside of the weapons chamber. At this point the head of the case could deform, and the primer pocket enlarge releasing the primer and the hot gasses back into the bolt face. But remember the case head has not been annealed, it is work hardened and resists the deformation to a large degree. A loose primer after firing is a warning sign. If the work hardened case head has deformed or enlarged enough to release the primer, something is seriously amiss. Overpressure is often (but not always) the culprit.
The retention of the primer can be especially problematic in semi-automatic firearms which utilise a fixed gas system. Ammunition built for such systems within the Mil/LE markets will often feature a primer which has been “staked” or crimped in. In these cases, the work hardening is not sufficient to retain the primer. This is the main reason that firearms designed to run at increased pressure and velocity (such as the Sig XM5 Next Gen rifle) require a specialised case to function. The Swiss P Hybrid case technology features a steel case head to resist deformation and retain the primer under increased pressure. Such features have not yet made it to the commercial market, but no doubt they will in the near future.