Polymer Cartridge Cases
Interesting news from America’s 2020 SHOT Show is the newly formed partnership between Sierra Bullets and polymer ammunition manufacturer True Velocity. They announced a competition-grade line of composite-ammunition using polymer-walled cases, claiming it capable of sub-moa accuracy and single-digit deviation in muzzle velocity. It will be available during 2020. Polymer cases have long been used for shotgun ammunition, and plastic-cased 7.62x51 training blanks were in military use in SA by the early 1960s. But we’ve not yet seen successful polymer-walled bulleted cartridges for bolt-action rifles.
True Velocity from Texas, has developed and produced composite metal-polymer cased ammunition for many years. The case head is steel alloy for strength and reliable extraction (extractor claws rip through polymer case rims). The steel case head has a polymer over-mould. A separate mould process forms the case shoulder and neck, and then the two parts are joined to create a complete case.
This manufacturing process affords uniformity in case wall thickness, with the added benefit that, in a given calibre such as the .308 Win, the internal dimensions of the case can be made to differ from those of standard drawn brass .308 Win cases in order to suit differing charges for achieving specific internal ballistics. Thus more efficient combustion can be achieved for greater consistency, and less powder is used to achieve a particular velocity.
Drawing a brass case begins with ‘cookie-cutting’ a disc from a sheet of brass. This coin-like disc is first stamped into cup form, rather like a large, thick-walled primer. A drawing process then forms it into an elongated hollow cylinder, blind at one end, its walls stretched longer and longer by means of high-pressure rams forcing the brass into a forming die. Manufacturing processes vary; the brass may undergo additional steps, such as annealing and washing after each stretching process as well as quality checks.
The case head is stamped into shape and the flash-hole punched through the web. Then the walls are tapered and shaped in stages to acquire the desired body, shoulder and neck dimensions. Brass is the favoured metal because it is malleable, strong enough, cheap and readily available. Brass cases can be reformed and reloaded; polymer cases cannot.
True Velocity claim their ammo has at least 30 advantages over brass, including 30% reduced weight, reduced heat-transfer to weapons, enhanced accuracy, consistent muzzle velocity, reduced muzzle flash, and more efficient powder combustion.
For most hunters and sport shooters, ammunition weight is of no concern, but for military use, weight limits the carrying capacity of individual soldiers, armoured vehicles, aeroplanes, helicopters and boats. Further up the line, it affects transportation capacity and costs. For the foot-soldier, lightweight ammunition affords more agility when carrying the same number of rounds, or more firepower for the same carry weight. The average combat soldier carries seven 30-round magazines – 210 rounds. Reduced heat-transfer will also enhance long-term barrel life. The military is therefore keen to invest in composite steel-polymer cased ammunition.
In late 2019, True Velocity’s 6.8mm composite-cased cartridge was selected
for the US Army’s Next Generation Squad Weapon modernisation programme, in conjunction with two other companies. At present, True Velocity’s ammunition, offered in 5.56x45 NATO, 7.62x51 NATO, .308 Winchester and .50 BMG, is available only to the US military. The company’s website indicates they’re also working with Norma to produce unique sniper ammunition for law enforcement agencies.
Successful military small-arms developments are usually adopted by the commercial industries. Initially, the new dual-branded Sierra-true Velocity ammunition for hunters and sport shooters will be available only in .308 Winchester loaded with Sierra’s 168- and 175-grain Matchking and Tipped Matchking bullets, and in 6.5mm Creedmoor with a 142-grain Matchking bullet. It will be interesting to see how hunters and sport shooters receive this ammo. Generally, recreational shooters tend to care more about accuracy and affordability than do the military. The question is whether the lower shipping and hauling costs of polymer-walled ammo will make it cheaper than brass-cased ammo, and if so, by how much? Given the wide variety of sporting cartridges in common use, and the benefits of hand-loading, it’s probably safe to say brass cases will be around for quite a while yet. – André Grobler