.408 Cheyenne Tactical

The .408 Cheyenne Tactical (or .408 CheyTac) is a specialized rimless, bottlenecked, centerfire cartridge 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 2,200 yards (2,000 m). It is hoped it will fill in a supposed market niche for long-range cartridges and is offered as a competitor to the most common military NATO long-range service cartridges .338 Lapua Magnum and the .50 BMG.

History
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 CheyTac cartridge casing 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.

Cartridge dimensions
The .408 CheyTac is neither officially registered with nor sanctioned by the Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives (CIP) or its American equivalent, Sporting Arms and Ammunition Manufacturers' Institute (SAAMI). 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 them is at one's 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 rifling twist rate for this cartridge is 330.2 mm (1 in 13 in meaning one 360 degree rotation (of the projectile) for every 13 inch the projectile travels forward through the rifled barrel), 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 Cheyenne tactical as parent case
The .408 CheyTac as a wildcat cartridge serves as parent for several other second generation wildcat cartridges.

By blowing out .408 CheyTac 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 CheyTac variant is the .408 Baer.

Besides changing the shape and internal volume of the parent cartridge case, wildcatters also can change the original caliber. Because the .408 CheyTac offers a large and sturdy, pressure resistant cartridge case it has become quite popular amongst wildcatters. With the .408 CheyTac as the parent case wildcatters have created .338 (.338 Snipe-Tac, 338/408 Baer), .375 (.375 CheyTac, .375-.408 CheyTac, .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 CheyTac chambering in their 2007 retail price list.

Projectiles (bullets)
Most .408 CheyTac factory ammunition uses solid projectiles or bullets rather than jacketed lead-core bullets, which are common to most other rifle bullets. The oldest factory .408 CheyTac ammunition uses bullets designed by Warren S. Jensen and originally produced by Lost River Ballistic Technologies. Currently (2009) these projectiles are produced by Jamison International, where they are turned on Swiss-type CNC lathes from solid bars of proprietary copper nickel alloy. 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.

Other manufacturers, such as Sierra, Rocky Mountain Bullet Company/Vigilance Rifles, GS Custom Bullets, Lehigh Bullets & Design, Lutz Möller and TTI Armory have developed lead-core or mono metal very-low-drag projectiles for the .408 CheyTac.

In 2007 Dr. John D. Taylor 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. The projectile has a mass of 370 grains (24.0 g). Claimed penetration of AR500 certified steel is 1 inch (2.54 cm) at 100 yards (91.5 m) and 1/2 inch (1.27 cm) at 775 yards (708.8 m).

In 2008 factory loaded .408 CheyTac 420-grain (27.22 g) lead core bullets with 955 copper jackets became available. The very-low-drag bullets are made by Rocky Mountain Bullets in Philipsburg, Montana and have a rebated boattail and a claimed ballistic coefficient (BC) of 0.874.

Factory .408 CheyTac ammunition is expensive, starting at around $7 per round with Jamison International 419-grain (27.15 g) very-low-drag projectiles.

Ballistic coefficient of the Jamison 419 gr projectile
Cheyenne Tactical claimed a Doppler radar-measured G1 ballistic coefficient (BC) for the Lost River Ballistic Technologies/Jamison International 419-grain (27.15 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 CheyTac projectile to average 0.945 to 2000 meters and it drops to the low 0.900s to 2800 meters. The .408 CheyTac 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 (27.15 g) is 0.945 over 3,825 yards (3,500 m). Jamison International states the G1 BC of this bullet on their website currently (2009) 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.

Balanced-flight/controlled-spin projectile
Lost River Ballistic Technologies (statement of Mr. Warren Jensen) stated that "the .408 CheyTac 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 directly 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,. 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. The patent does not account for normally occurring differences in air density. More about balanced flight can also be found in the CheyTac Information Papers.

The Balanced Flight/Controlled Spin Projectile bullet patent has been questioned/disputed by the German physicist Lutz Möller. Mr. Möller realized balanced flight has to do with the nature of spin stabilized flight and scale of parameters. There were projectiles produced prior to the patent that remain stable through the transonic flight regime. This is a consequence of the spin deceleration and forward deceleration of the projectile being similar enough not to cause undesirable precession and yaw during the transonic flight phase. The main parameter for achieving stable transonic transition is controlling the drag coefficients (Cd) and forward velocity loss around Mach 1 and to a lesser degree controlling the spin deceleration. In other words, any bullet with appropriate drag behaviour around Mach 1 and mass (distribution) will do exactly what the balanced flight projectile patent states.

Performance
The cartridge delivers accurate (sub-MOA) performance from a sniper rifle platform. The Lost River Ballistic Technologies/Jamison International 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.

Supersonic range
For a typical .408 CheyTac chambered gun, shooting 27.15 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 CheyTac 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 CheyTac brass is possible, but the bullets have to be very long and the normal cartridge overall length has to be exceeded. The common .375 CheyTac 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 CheyTac 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). This bullet has since exhibited dynamic stability problems and is no longer produced. The .375 Viking bullet had an overall length of 70 mm (2.756 in) and derived its anticipated 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), were to have been equipped with custom made 762 mm (30 in) long 203 mm (1:8 in) twist rate barrels.

In theory, Mr. Möller calculated that a typical .375 CheyTac chambered gun, shooting his now defunct 26.44 gram (408 gr) .375 Viking bullets (claimed G1 BC = 1.537) at 870 m/s (2854 ft/s) muzzle velocity, would have a supersonic range of 3090 m (3380 yd) under International Standard Atmosphere sea level conditions (air density ρ = 1.225 kg/m³). However, field testing of this projectile proved it to be completely unstable, and useless at any velocity, or range. This was established in February 2009 by Terry Holstine, an Oklahoma State Trooper, who is the only person to ever have fired the Viking. It would appear that Mr. Möller did not test this projectile prior to public release whereby the dynamic instability would have been discovered. The ballistic coefficient of this bullet has never been verified, therefore the supersonic range using this projectile cannot be determined due to the instability and unknown ballistic coefficient regardless of the claims made.

Chambering availability
The .408 Cheyenne Tactical chambering is offered for these factory rifles:
 * BCM Europearms S.a.s.
 * CheyTac Intervention rifle series - CheyTac LLC.
 * E.D.M. Arms XM04
 * PGWDTI Timberwolf .408 CheyTac and .416 PGW rifle - Prairie Gun Works Defence Technologies Inc.
 * Lawton Machine LLC. (Lawton Rifle Barrels)
 * Lobaev Sniper Rifle
 * Grande Armeria Camuna precision rifles
 * RND Manufacturing, Inc.
 * Tactilite T1 (single-shot) AR-15 .408 Chey Tac upper
 * THOR XM408
 * Vigilance Rifles VR1
 * Desert Tactical Arms HTI

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 oriented 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 for a particular rifle optimized ammunition is considered normal. Only expert marksmen can make use of this extreme accuracy potential.