Metal treatment – Stock – Chromium – molybdenum – or tungsten base
Reexamination Certificate
2001-08-06
2003-03-04
Mai, Ngoclan (Department: 1742)
Metal treatment
Stock
Chromium, molybdenum, or tungsten base
C148S315000, C148S442000, C420S430000, C420S444000, C420S590000
Reexamination Certificate
active
06527880
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to metallic shot with improved properties for use in hunting or shooting, and to other articles traditionally made of lead alloys.
BACKGROUND OF THE INVENTION
Because of the use of traditional lead (Pb) shot has been outlawed for waterfowl hunting in the U.S., Canada, U.K. and other countries, much effort has been devoted to identifying a suitable substitute. To be fully satisfactory, alternative shot must possess the following attributes:
a) The material should have density similar to that of lead (Pb) shot, typically 11.0 g/cm
3
.
b) The material must not cause physiological problems in wildlife which may ingest spent shot from the ground or water.
c) The material must not cause significant damage to shotgun barrels.
d) Shot must possess sufficient strength, rigidity and toughness to adequately withstand “set-back” forces associated with firing and to penetrate the target effectively without shattering or excessively deforming.
e) For purposes of game law enforcement, shot material should preferably be magnetic to easily differentiate it from illegal lead shot.
f) Material used for shot must be economical to obtain and fabricate into spherical product.
None of the alternative shot types currently available conforms to all of the above criteria. Current products in the USA include shot made of steel, bismuth alloy, iron-tungsten alloy and tungsten-polymer composite. Each of these will be reviewed and critiqued in the following discussion, followed by a review of other prior art which has not yet become commercialized.
Steel Shot
The most widely used alternative shot is carbon steel, in spite of the fact that its density is quite low (about 7.9 g/cm
3
) in comparison with that of lead shot (about 11.0 g/cm
3
). Inarguable principles of physics and engineering establish that an object of lower density, when moving through a fluid (such as air), will carry less energy at any given velocity, and experience more rapid loss of velocity (due to drag forces) than an object of higher density of the same size and shape. Shotshell manufacturers have employed special powders to increase steel shot velocity, in an attempt to ameliorate its inferior ballistic properties. The “hotter” powders unfortunately create higher pressures within the gun barrel. Safety considerations have therefore prompted shotshell manufacturers to recommend that steel shells only be fired in certain types of modern, high-strength shotguns.
There is also a significant negative impact of steel shot on the very same wildlife which the outlawing of lead is intended to preserve. The inferior ballistics of steel shot, in the hands of the general public, has resulted in higher rates of “crippling” shots. The January, 1997 issue of
American Hunter
refers to “Goose hunters accustomed to shooting traditional lead shot tend to attempt to shoot waterfowl at the same distances as they have always considered to be “in range.” Another approach taken by steel shotshell manufacturers has been to simply substitute larger steel shot for traditional lead shot sizes, in order to provide equivalent mass.
This practice has the obvious disadvantage that there are fewer shots in any given shell. The “pattern density” of the cloud of shot is lower at any given distance from the point of firing. This sparse pattern again increases the probability that birds will be crippled, rather than harvested for consumption. In summary, a statement by the Shooting Editor of
Outdoor Life Magazine
, Jim Carmichel, is quoted: “. . . steel shot has generally been considered only a quick fix in the search for the ultimate shot pellet.” (April, 1997 issue, page 73).
Bismuth Shot (U.S. Pat. No. 4,949,644 to Brown)
Bismuth alloy shotshells are currently marketed in the USA at approximately three times the cost of steel shells, an indication of how desperate consumers are to obtain improved performance. Unfortunately, bismuth alloys are not equivalent to lead in density (about 9.4 g/cm
3
vs. 11.0 g/cm
3
), although somewhat more dense than steel (7.9 g/cm
3
). In addition to this shortcoming, bismuth alloys are inherently brittle and therefore tend to fracture and disintegrate upon impact (January, 1998 issue of
Gun Tests
). As fracture surfaces form in the shot, energy is lost which would otherwise be available to enhance penetration of the target. In this instance, it is even likely that all the increased energy gained by having higher density than steel is lost as fracture occurs. Finally, it should be noted that bismuth is non-magnetic and cannot be readily distinguished from illegal lead shot by game officers in the field.
Iron-Tungsten Shot (U.S. Pat. Nos. 5,264,022, 5,527,376 and 5,713,981 assigned to Teledyne Industries, Inc.)
A more recent product which began to be marketed in the USA in 1997 is a shotshell containing binary iron-tungsten alloy shot (60%Fe-40%W, by weight). Because the Fe—W is very hard (about Rockwell C50), and therefore must be ground with ceramic abrasives (alumina, silicon-carbide, diamond, etc.), particles of which become imbedded in the shot surface, this type of shot will result in severe damage in all gun barrels unless the shot is encapsulated in a special “overlapping double-wall” plastic shot-cup of heavy construction. Even with this precautionary design, the manufacturer prints a clear message on each box of product disclaiming any responsibility for gun barrel damage or personal injury. Although controversial, one current theory is that it is possible for a few shot to rebound forward out of the plastic cylinder upon firing and to thereby contact the unprotected steel barrel. The consequences of forming longitudinal scratches on the barrel are that stresses produced by the expanding explosive gases will be concentrated in the regions around the scratches. A primary concern is that these stresses may be sufficiently high to cause catastrophic bursting of the barrel.
Whether adequately protective or not, the special plastic shot-cup (or “wad”) creates another significant problem. The wad must be made of plastic tubing so thick as to make it impossible to load quantities of shot equivalent to those of traditional lead shells. For example, Fe—W shells of 2¾-inch length for 12-gauge guns contain only 1.0 ounce of shot versus 1⅛ to 1¼ ounces in corresponding lead or steel shells. The deficient pellet numbers result in correspondingly sparse pattern densities, the same problem encountered in substituting larger steel shot for traditional lead sizes, as mentioned previously.
Although more dense than bismuth shot, Fe—W shot currently marketed is still considerably less dense than lead shot (about 10.2-10.5 g/cm
3
vs. 11.0 g/cm
3
). When this fact is combined with the lower pattern densities, the purported advantages of Fe—W shot over steel shot become questionable.
Finally, problems associated with manufacturability, and their adverse effects on product cost, are relatively severe. The constituent phases in Fe-W alloys cause the shot to be so hard and brittle as to be impossible to forge or swage these alloys into rods, or even to shape them compressively into spheres. Although the referenced patents claim Fe—W shot can be made by casting, the inherent brittleness and high melting temperatures of these alloys caused cracking to occur during rapid cooling. Cracking also plagued the process of compressive grinding, which was tried as a means of rounding the generally asymmetrical shot. Consequently, the shot actually being produced and marketed must be made by an expensive powder metallurgical method. Even with this approach, only larger shot sizes (“BB” 0.180-inch-diameter, and “#2” 0.150-inch-diameter) are being produced at present. This is due to the fact that powder processing costs increase exponentially as shot sizes decrease. Furthermore, the fragility of compaction tooling becomes a limiting factor as shot size decreases. Shot sizes #4 (0.130-inch), #5 (0.120-inch), #6 (0.110-inch) and #7½ (0.095-inch), traditionally preferred
Kolisch & Hartwell, P.C.
Mai Ngoclan
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