The present invention relates to firearm cartridges and percussion nipples, and more specifically to the flash hole design of either a cartridge used in a modern firearm, or the nipple used in a percussion black powder firearm, and to a specialized drill used to produce these shapes.
An ammunition cartridge typically consists of a brass or steel case having a main chamber filled with a propellant such as smokeless powder and a bullet seated in the front of the chamber. A recessed pocket is provided in a rear face of the cartridge to house a primer. One or two flash holes is/are punched in the wall separating the primer pocket from the chamber containing the propellant. When the primer is detonated, hot gas shoots through the flash hole(s) and into the chamber to ignite the propellant. There are two styles of cartridges. A “Boxer” cartridge has one central flash hole approximately 0.080″ to 0.090″ in diameter and a primer pocket approximately 0.175″ in diameter. A “Berdan” cartridge has two diametrically opposing flash holes approximately 0.050″ in diameter.
The flash hole is typically punched in the primer pocket from outside the cartridge towards the interior. This manufacturing operation occasionally results in a burr on the inside edge of the flash hole. Competitive shooters remove the burr with a special drill called “flash hole uniformer” to counter bore the flash hole from the inside of the cartridge.
As illustrated in
As illustrated in
The present invention constitutes an improvement in a firearm cartridge and a percussion nipple of a black powder rifle, and a tool for providing these improvements. The cartridge may have a conventional upper chamber adapted to house gunpowder and a bullet, a lower pocket adapted to house a primer, and a flash hole extending there between. The lower pocket has a ledge disposed generally perpendicularly to the flash hole. The improvement comprises providing a flash hole that is formed with a perimeter wall having a convex curvature adjacent to the ledge of the primer pocket. In the percussion nipple, an elongated bore is formed with relatively larger and smaller diameter portions connected by a shoulder, and the improvement comprises providing the shoulder with a convex curvature adjacent to the larger diameter portion of the bore. The tool has a tip formed with a concave curved surface corresponding to the convex curvature of the flash hole perimeter wall on the cartridge or to the convex curvature of the shoulder in the elongated bore of the percussion nipple.
In incompressible flow (or gas flow at subsonic speeds of less than about Ma 0.7, where gas can effectively be modeled as incompressible), a constriction in pipe diameter will yield an increase in velocity, since mass flow rate along the pipe is theoretically constant. However, where there is compressible, or supersonic flow, as obtained from the detonation of a high explosive primer, a sudden constriction in a conduit will cause a choking condition or resistance that limits the flow of ignition gas through the constriction. It is believed that the amount of such resistance depends upon the shape of the constriction, particularly the inlet of the constriction. One that is perpendicular to the axis of the conduit will cause a loss that is greater than a chamfered inlet which, in turn, will cause flow loss that is greater than a rounded inlet constriction. As these losses are examined in the context of supersonic flow of ignition gas in a firearm cartridge, the flash hole may be seen as a supersonic convergent nozzle, where the negative effect of the change from a large diameter to a small diameter on fluid flow may be mitigated by the shape of the constriction.
In order to allow the greatest and fastest flow of gas from the primer chamber to the powder chamber, it is therefore desired that the flash hole create the least amount of back pressure possible. To this end, it is proposed that the generally perpendicular intersection of the flash hole perimeter wall 32 and the primer pocket upper ledge 30 illustrated in
Further, and depending on the gas parameters, other profiles which are not round might be utilized to advantage. Parabolic, elliptical, power series, Von Karmen, Haack, or many other shapes might be optimum. And again, any of these profiles might swing through a full 90 degrees, but might also only swing through a reduced angle. As illustrated in
The same or similar principle may be applied to flash nipples for black powder rifles and pistols.
The third element to this patent is the shaped drill which is so shaped that when it is used to drill a hole, the tip profile is one of the aforementioned shapes of a parabola, ellipse, Von Karmen profile, Haack profile, or power series profile.
A further embodiment of the flash hole design can be seen in the application of this invention to the percussion nipples of a black powder rifle. In these rifles, a tubular hollow nipple protrudes from the breech, wherein the powder is packed behind the bullet. Instead of a primer which is inserted into the primer pocket of a brass case, as described above, a percussion cap is placed over the tip of the nipple, and is struck directly with the hammer. The nipple thus acts as the anvil, such that the compressive shock generated between the hammer and the anvil will detonate the primer compound. The primer nipple is designed such that there is initially a large bore, allowing the hot gases from the primer detonation to flow relatively unobstructed, allowing fast propagation of the gas. However, at the end of the nipple, there is a reduction of diameter of the bore. This is necessary, so that once the powder in the breech is ignited, the high pressure in the barrel does not cause significant back-flow out the nipple, leading to a loss of pressure behind the bullet, or potentially blowing back the hammer, causing it to be damaged. It is desired that this smaller bore be of generally short length, in order to minimize the flow restriction of the primer gasses in the direction passing into the breech where the powder is stored. A low flow restriction into the breech is desired in order to get maximum ignition reliability. Typical for nipple construction is that the large bore, on the order of 0.075″ to 0.100″, is drilled to partial depth, and then a smaller bore, on the order of 0.025″ to 0.035″ is drilled through. Using this manufacturing technique, the transition of the large bore into the small bore will have a steep chamfer, or conical transition, with an included angle typical of a drill bit used for machining. That is to say, an included angle of 118 degrees or 135 degrees.
Applying this invention to a black powder flash nipple, it can be seen that replacing the conical transition from the large bore to the small bore, having a 135 or 118 degree included angle, with a smooth transition, would improve flow. This smooth transition could be in the form of a DeLaval supersonic nozzle design, it could also use a trumpet shape which has a sudden onset of the converging section, but then comes tangent to the small diameter bore. As described above, the transition could use a surface based on the revolution of a circular arc, or a parabolic, or elliptical arc, or an arc from some other mathematically derived curve.
It is contemplated that the foregoing convex perimeter wall of the flash hole and the convex shoulder of the flash nipple may be formed with a drill or end mill. The tip of the tool would have a concave curved surface which corresponds to any of the profiles described above, such that the drill can be used to drill or mill, or otherwise cut the various flash hole profiles into a cartridge or into a flash nipple.
Number | Date | Country | |
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61858219 | Jul 2013 | US |