The .408 Cheyenne Tactical (or .408 Chey Tac) is a specialized rimless bottlenecked centerfirecartridge for military long-range sniper rifles that was developed by Dr. John D. Taylor and machinist William O. Wordman. It was designed with a possible military need in mind for a cartridge for both the anti-personnel, anti-sniper and anti-materiel role with a (supersonic) precision range of 2,187 yards (2,000 m). It is hoped it will fill in a supposed market niche as an intermediate between the existing military long-range cartridges .338 Lapua Magnum and the .50 BMG.
The .408 Cheyenne Tactical is based on the .400 Taylor Magnum, which is based on a modified .505 Gibbs, necked down to 0.408 inches (10.36 mm). The .505 Gibbs is an old English big game cartridge that was designed to accommodate 39,160 psi (270 MPa) pressure. One of the disadvantages to these old cartridge cases intended for firing cordite charges instead of modern smokeless powder is the thickness of the sidewall just forward to the web. During ignition, the cartridge's base, forward to the bolt face, is not supported. The case is driven back against the bolt face which results in the stretching of the case, particularly the sidewall immediately forward of the web. When the sidewall resists the outward expansion against the chamber, the pressure stretches the case thereby increasing its length resulting in the sidewall becoming thinner at that stretch point.
In the .408 Chey Tac cartridge case design particular attention was directed toward thickening and metallurgically strengthening the case's web and sidewall immediately forward to the web to accommodate high chamber pressures. In modern solidhead cases, the hardness of the brass is the major factor that determines a case's pressure limit before undergoing plastic deformation. Lapua Ltd. solved this problem when they used the .416 Rigby as the parental case to the .338 Lapua Magnum. They created a hardness distribution ranging from the head and web (hard) to the mouth (soft) as well as a strengthened (thicker) case web and sidewall immediately forward of the web. This method results in very pressure resistant cases.
The .408 Chey Tac is neither officially registered with nor sanctioned by CIP (Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives) or its American equivalent, SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute). Such cartridges are generally known as wildcats. Wildcat cartridges have no official dimensions nor other performance related specifications. Stated data about wildcats is always informal and using it is at ones own risk.
According to the QuickLOAD interior ballistics predictor program the cartridge case capacity of the .408 Cheyenne Tactical is about 159 grains (10.32 ml) of water. The exterior shape of the case was designed to promote reliable case feeding and extraction in bolt action rifles, under extreme conditions. QuickLOAD states the maximum operating piezo pressure at 63,816 psi (440 MPa) and shows that this is a wildcat cartridge. The common riflingtwist rate for this cartridge is 330.2 mm (1 in 13 in), eight 0.4080 in grooves, 0.4010 in bore 0.050" land width, square cut, no taper and the primer type is large rifle.
The .408 Chey Tac as a wildcat cartridge serves as parent for several other second generation wildcat cartridges.
By blowing out .408 Chey Tac factory cases the wildcatter generally hopes to gain extra muzzle velocity by increasing the case capacity of the factory parent cartridge case by a few percent. Practically there can be some muzzle velocity gained by this method, but the measured results between parent cartridges and their 'improved' wildcat offspring is often marginal. An example of a blown out .408 Chey Tac variant is the .408 Bear.
Besides changing the shape and internal volume of the parent cartridge case, wildcatters also can change the original caliber. Because the .408 Chey Tac offers a large and sturdy, pressure resistant cartridge case it has become quite popular amongst wildcatters. With the .408 Chey Tac as the parent case wildcatters have created .338 (.338 Snipe-Tac, 338/408 Bear), .375 (.375 Chey Tac, .375-.408 Chey Tac, .375 Snipe-Tac, .375 SOE), .416 (.416 PGW) and .510 (.510 Snipe-Tac) caliber variants. The 0.375 inch (9.525 mm) caliber variants currently (2007) seem to attract the most attention. The company that designed the CheyTac Intervention rifle CheyTac, LLC. itself offers rifles in a .375 Chey Tac chambering in their 2007 retail price list.
All current .408 Chey Tac factory ammunition uses solid projectiles or bullets rather than jacketed lead-core bullets, which are common to most other rifle bullets. The factory .408 Chey Tac ammunition uses bullets produced by Lost River Ballistic Technologies, where they are turned on Swiss-type CNC lathes from solid bars of proprietary coppernickel alloy. The projectiles are then sized through pressure dies to make their outside diameters more uniform; the factory claims their diameter is accurate to "one 50 millionth" but does not provide a unit of measurement with this claim, making it somewhat vague. One noted downside to the use of solid mono metal projectiles is that they tend to increase the fouling left in the rifle barrel after they are fired. Since the bullets are harder and more abrasive than the gilding metal jackets of normal jacketed bullets, they are made slightly "undersized" so that they may be gripped effectively by the lands of the barrel's rifling. This inevitably reduces the seal of the bullet in the barrel, allowing hot gunpowder gases to reach the sides of the projectile, vaporizing some of the material and depositing it in the bore.
Dr. John D. Taylor recently (2007) designed a new class of armor-piercing projectiles (patent pending). The cartridge is called the .408 CheyCorey and in this configuration, it outperforms the .50 AP (both black and silver tips) cartridge against armor steel and titanium.
Factory .408 Chey Tac ammunition is expensive, starting at around $5 per round with Lost River Ballistic Technologies 419-grain (26.95 g) very-low-drag projectiles.
Ballistic Coefficient of the Lost River 419-grain projectileEdit
Cheyenne Tactical claimed a Doppler radar-measured G1 ballistic coefficient (BC) for the Lost River Ballistic Technologies 419-grain (26.95 g) bullet of roughly 0.934, though this number has been disputed by a number of knowledgeable sources. Extreme Firearms claimed the G1 BC of the same .408 Chey Tac projectile to average 0.945 to 2000 meters and it drops to the low 0.900s to 2800 meters. The .408 Chey Tac projectile remains supersonic up to 2,300 yards (2,100 m) according to Extreme Firearms. CheyTac LLC claims that the 26.95 gram (419 gr) projectile has a supersonic range of 2,200+ yards (2,011+ m) at 'standard air conditions'. The average ballistic coefficient of the 419 grain (26.95 g) is 0.945 over 3,825 yards (3,500 m). Lost River Ballistic Technologies states the G1 BC of this bullet on their website currently (2007) at 0.940.
The above variations can be explained by differences in the ambient air density used for these BC statements or differing range-speed measurements on which the stated G1 BC averages are based. The BC changes during a projectiles flight and stated BC's are always averages for particular range-speed regimes. Some more explanation about the transient nature of a projectile's G1 BC (it rises above or gets under a stated average value for a certain speed-range regime) during flight can be found at the external ballistics article. This article implies that knowing how a BC was established is almost as important as knowing the stated BC value itself.
Lost River Ballistic Technologies (statement of Mr. Warren Jensen) stated that the .408 Chey Tac is the first bullet/rifle system that utilizes what they call a balanced flight projectile. To achieve balanced flight the linear drag has to be balanced with the rotational drag to keep the very fine nose (meplat) of the bullet pointed direct into the oncoming air. It should result in very little precession and yaw at extreme range and allows accurate flight back through the transonic region. This is hard to achieve for small arms projectiles. Mathematically you are at a great disadvantage trying to achieve balanced flight with a lead core non mono metal bullet. The rotational mass/surface area ratio is too high.
The balanced flight projectile patent can be found at the US Patent Office, Controlled spin projectile, US PAT No. 6,629,669. According to the patent a projectile engraved and launched according to the teachings of the patented invention should decelerate from supersonic flight through transonic to subsonic in a stable and predictable manner effective to a range beyond 3000 yards (2,743 m). It implies that amongst several other preconditions the rifle barrel has to have specific rifling dimensions to achieve a desired amount of axial air drag on the bullets surface, which reduces the bullets spinrate to achieve balanced flight. More about balanced flight can also be found in the Chey Tac Information Papers.
The Balanced Flight/Controlled Spin Projectile bullet theory has been questioned/disputed by the German physicist Lutz Möller.
The cartridge delivers accurate (sub-MOA) performance from a sniper rifle platform. The Lost River Ballistic Technologies 419-grain (26.95 g) very-low-drag bullet is the standard, long-range sniping load.
Lost River Ballistic Technologies also designed a 305 grain (19.76 g) bullet for the Battlefield Domination Round (BDR). The BDR is loaded with a 305 grain (19.76 g) bullet (claimed G1 BC = 0.612) and is intended for short and medium range application using the point blank range aiming method.
For the above muzzle velocity of 2,900 ft/s (884 m/s) out of a 30 inch (762 mm) long barrel QuickLOAD predicts 68,200 psi (470 MPa) maximum piezo chamber pressure. This is a very high maximum chamber pressure value for rifle cartridges.
For the muzzle velocity of 2,900 ft/s (884 m/s) out of a 35 inch (889 mm) long barrel QuickLOAD predicts a less extraordinary 61,700 psi (425 MPa) maximum piezo chamber pressure. One could suspect that the muzzle velocity of 2,900 ft/s (884 m/s) was reached with the help of a long barrel.
For the on YouTube mentioned 2,750 ft/s (838 m/s) load out of a 30 inch (762 mm) long barrel QuickLOAD predicts 56,000 psi (386 MPa) maximum piezo chamber pressure. This is a safe maximum chamber pressure for many modern rifle cartridges.
For a typical .408 Chey Tac chambered gun, shooting 26.95 gram (419 gr) Lost River Ballistic Technologies bullets (claimed G1 BC = 0.940) at 884 m/s (2900 ft/s) muzzle velocity, the supersonic range would be ≈ 1930 m (2110 yd) under International Standard Atmosphere sea level conditions (air density ρ = 1.225 kg/m³).
For a typical .375 Chey Tac chambered gun, shooting 24.30 gram (375 gr) Lost River Ballistic Technologies bullets (claimed G1 BC = 1.02) at 930 m/s (3050 ft/s) muzzle velocity, the supersonic range would be ≈ 2230 m (2440 yd) under International Standard Atmosphere sea level conditions (air density ρ = 1.225 kg/m³).
Improvement beyond this standard while still using standard .375 Chey Tac brass is possible, but the bullets have to be very long and the normal cartridge overall length has to be exceeded. The common .375 Chey Tac 292 mm (1:11.5 in) rifling twist rate also has to be tightened to stabilize very long projectiles. The use of such a .375 Chey Tac based cartridge demands the use of a custom or customized rifle with an appropriately cut chamber and a fast-twist bore. An example of such a special .375 caliber extreme range bullet is the German CNC manufactured mono-metal 26.44 gram (408 gr) .375 Viking (G1 BC ≈ 1.537 – this Ballistic coefficient (BC) is calculated by its designer, Mr. Lutz Möller, and not proven by Doppler radar measurements). The .375 Viking bullet has an overall length of 70 mm (2.756 in) and derives its exceptional low drag from a radical LD Haack or Sears-Haack profile in the bullet's nose area. Rifles chambered for this wildcat cartridge, with a cartridge overall length of 119 mm (4.685 in), have to be equipped with custom made 762 mm (30 in) long 203 mm (1:8 in) twist rate barrels.
For a typical .375 Chey Tac chambered gun, shooting Lutz Möller’s 26.44 gram (408 gr) .375 Viking bullets (claimed G1 BC = 1.537) at 870 m/s (2854 ft/s) muzzle velocity, the supersonic range would be ≈ 3090 m (3380 yd) under International Standard Atmosphere sea level conditions (air density ρ = 1.225 kg/m³).
.408 Chey Tac ballistic comparison with other long-range sniper cartridges
Several high quality large tactical and match (semi) custom bolt actions were designed for the .408 Cheyenne Tactical cartridge and are currently (2007) becoming available. These (semi) custom bolt actions are used with other high grade rifle and sighting components to build custom sporting and target rifles. Such rifles are ordered by long-range accuracy orientated shooters and built by specialized, highly skilled gunsmiths and can cost thousands of dollars. When built to expectation such rifles are very accurate - 0.5 MOA or better consistent accuracy with for a particular rifle optimized ammunition is considered normal. Only expert marksmen can make use of this extreme accuracy potential.