.408 Chey Tac

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The .408 Cheyenne Tactical (or .408 Chey Tac) is a is a specialized rimless bottlenecked centerfire cartridge for military long-range sniper rifles that was developed by Dr. John D. Taylor with the assistance of 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-material role with a precision range of 2000 meters (2187 yards). 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.

Contents

[edit] History

The .408 Cheyenne Tactical is based on the .400 Taylor Magnum, which is based on a modified .505 Gibbs, necked down to 10.36 millimeters (.408 in). The .505 Gibbs is an old English big game cartridge that was designed to accommodate 270 MPa (39160 psi) 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 resulted in a very pressure resistant case.

[edit] Cartridge Dimensions

The .408 Chey Tac is neither officially registered with nor sanctioned by CIP (Commission Internationale Permanente Pour L'Epreuve Des Armes A Feu Portative) or its American equivalent, SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute). Such cartridges are generally known as wildcats. Wildcat cartridges have no offical 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 10,32 ml (158.9 grains) of water. The exterior shape of the case was designed to promote reliable case feeding and extraction in bolt action rifles and machineguns alike, under extreme conditions. QuickLOAD states the maximum operating piezo pressure at 440 MPa (63816 psi) and shows that this is a wildcat cartridge.

[edit] The .408 Cheyenne Tactical as Parent case

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. A reason to change the original caliber can be to comply with a minimal permitted caliber or bullet weight for the legal hunting of certain species of game. Because the .408 Chey Tac offers a large and sturdy, pressure resistant cartridge case that can relatively easily be reloaded with primers, powder and bullets and hence be reused several times it has become quite popular amongst wildcatters. With the .408 Chey Tac as the parent case wildcatters have created .338 (.338 Snipe-Tac), .375 (.375 Chey Tac, .375-.408 Chey Tac, .375 Snipe-Tac, .375 SOE), .416 (.416 PGW) and .510 (.510 Snipe-Tac) caliber variants. The 9.525 mm (.375 in) 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.

[edit] Projectiles (Bullets)

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 copper nickel 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 misleading or 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 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 and TTI Armory, have developed lead-core or mono metal projectiles for the .408 Chey Tac, but all are currently (2007) in short supply.

Factory .408 Chey Tac ammunition is prohibitively expensive for most civilian long-range shooters, starting at around $5 per round with Lost River Ballistic Technologies 26.95 gram (419 gr) projectiles.

Dr. Taylor recently 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.

[edit] Ballistic Coefficient (G1 BC) of the Lost River Ballistic Technologies 26.95 gram (419 gr) Projectile

Cheyenne Tactical claimed a Doppler radar-measured G1 ballistic coefficient (BC) for the Lost River Ballistic Technologies 26.95 gram (419 gr) 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 reaches the speed of sound at 2100 meters (2297 yards) according to Extreme Firearms. CheyTac LLC claims in its Chey Tac Information Papers (2007) that the 26.95 gram (419 gr) projectile has a supersonic range of 2011+ meters (2200+ yards) at standard air conditions. The average ballistic coefficient of the 26.95 gram (419 gr) is 0.945 over 3500 meters (3828 yards). 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.

[edit] Balanced Flight / Controlled Spin Projectile

Lost River Ballistic Technologies (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 2743 meters (3000 yards). It implies that amongst several other preconditions the rifle barrel has to have specific rifling dimensions to achieve balanced flight. More about balanced flight can also be found in the Chey Tac Information Papers.

[edit] Performance

The cartridge delivers accurate (sub-MOA) performance from a sniper rifle platform. The Lost River Ballistic Technologies 26.95 gram (419 gr) Very Low Drag bullet is the standard, long-range sniping load.

The Lost River Ballistic Technologies 19.76 gram (305 gr) (claimed G1 BC = 0.612) bullet is used as a "close-range" (i.e., under 1000 yards) fast-targeting round. Because the 19.76 gram (305 gr)projectile has a 1.0 second flight time to 1000 yards and a maximum ordinate of approximately +40 inches above the crosshairs with a 1000 yard zero, a sniper using this round can zero his rifle for 1000 yards and aim mid-thigh on any enemy between 100 and 1000 yards, and be assured of a generally chest-level impact on the target without making any adjustments to the riflescope.

[edit] Muzzle Velocity

  • 19.76 gram (305 gr) Copper Nickel Solid: 993 m/s (3258 ft/s)
  • 26.95 gram (419 gr) Copper Nickel Solid: 884 m/s (2900 ft/s)

For the above muzzle velocity of 884 m/s (2900 ft/s) out of a 762 mm (30 in) long barrel QuickLOAD predicts 470 MPa (68167 psi) maximum pieze chamber pressure. This is a very high maximum chamber pressure value for rifle cartridges.

[edit] See also

[edit] External Links and Sources

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