So what is this swaging process? It is generally a cold-formed forging process (i.e.) the work is usually done at room temperature, without heating the metal. It consists of a hard metal die which has a cavity of the desired shape of the bullet inside it. Lead or any other material is inserted into the die's cavity and then it is put under pressure by means of a metal punch which is forced into the die under pressure until it reaches a preset depth. The punch pushes the lead material (or other bullet material) into the shape of the cavity. The pressure to the punch may be applied by a manual press, a hydraulic press, repeated hammer blows, or by using a threaded punch that is screwed on. For most industrial-style manufacturing, a hydraulic press is used and the pressure applied is in the range of a few tons. The pressure applied depends on the hardness of the bullet material, its ductility, shape of the bullet etc. After the bullet material has been shaped, the punch is then removed from the die, the die is opened and the swaged item is removed. This process allows for uniform density and repeatability of process with very high accuracy.
If a bullet made of multiple materials is desired (e.g. a jacketed bullet, tungsten tipped bullet or a tracer bullet), then it can be done in multiple swaging steps, i.e. insert the first material into the die and apply a punch to make the first layer, then insert more materials and then apply the punch again for the second layer and so on.
This technique has several advantages over casting bullets. In casting, since the molten metal shrinks when it solidifies, the mold must be slightly larger than the desired size. Therefore it is harder to control the size of the final product. Since swaging happens at room temperature, the swaging die is the exact size of the bullet desired and therefore produces a more accurate-sized bullet. Swaging can be used to make compound bullets and jacketed bullets made of multiple materials, whereas casting can make bullets that are only composed of one material. Cast bullets may have defects such as air bubbles and cracks, whereas swaged bullets do not have these defects. Swaging can also be used to make non-metallic bullets, such as plastic bullets.