This invention relates to an addition to every kind of propellant effective for removing copper deposits from the inside surfaces of a gun barrels. More particularly, a composite addition has a pulverized decoppering agent mixed into the propellant.
Most guns have a barrel with a rifled internal bore that imparts a stabilizing spin on an expelled projectile. The internal bore may be coated with a hard facing material, such as chromium, to minimize erosive wear increasing the number of projectiles that may be fired from the gun.
The typical large caliber projectile has a diameter slightly less than the diameter of the internal bore. One or more obturator, or rotating, bands gird the circumference of the projectile. At the bands, the diameter of the projectile is slightly larger than the internal diameter of the gun barrel. When the projectile is expelled, the rotating band is engraved by the rifling, contacting the rifling throughout the length of the tube imparting the projectile with a stabilizing spin. Projectiles for rifles and pistols normally do not have a rotating band but they very often have a jacket made of tombac, which is engraved by the rifling.
The gun barrel is manufactured from a material such as steel and sometimes coated with a hard material such as a chromium facing. The gun barrel is harder than the rotating band or jacket, which is typically copper or a copper alloy. As a result, a portion of the copper from the rotating band or the jacket is deposited on the rifling inside the gun barrel. This copper deposition referred to as "copper fouling" can affect the ballistics of the projectile and major fouling can prevent the projectile from being inserted and seated, positioned in the barrel prior to firing, properly.
Copper fouling is currently a major problem for large artillery weapons, such as 155-millimeter howitzers, and is also noted in small and medium caliber cannons, such as 20-millimeter canons. It has become a bigger problem in rifles today because of the use Of the lead-free primers. Before the primer contained lead and that small amount of lead worked as a decoppering agent. The current solution to copper fouling is including a decoppering agent into the propellant. The decoppering agent removes the copper without damaging the gun barrel of the rifling.
A common decoppering agent is a sheet of lead foil deposited between the propellant and the projectile. On ignition of the propellant charge, the lead is vaporized and diffuses into the copper. The resultant alloy is brittle and easily shattered. The combination of the heat generated by the burning propellant and the mechanical movement of the propellant gases separates the brittle lead/copper alloy from the surface of the barrel. The fractures debris is swept from the muzzle of the gun with the propellant gases.
A second theory as to why the lead foil is effective as a decoppering agent is that the heat generated by the burning propellant melts the lead foil. Liquid lead contacts the copper deposition and dissolves the copper, the copper bearing lead solution is expelled as a liquid from the muzzle with the propellant gases.
While metallic lead and lead compounds are effective decoppering agents, the materials are toxic to humans working around the weapons. There is a need for a lead free decoppering agent
Among the lead free decoppering agents that have been proposed are bismuth, bismuth subcarbinate (BiCO, sub 3), tin and tin alloys. Bismuth compounds are very brittle and even metallic bismuth cannot be rolled into a thin foil, but the alloys are very expensive and less effective as a decoppering agent.
There are also solutions described in patent document No
There remain, therefore a need for a method to effectively introduce a lead free decoppering agent into the propellant. To include the decoppering agent in the propellant composition makes it easier and cheaper when having to add a special made decoppering additive to the charge.
According to the present invention, a decoppering agent for a propellant, which comprises essentially of a lead free pulverized additive, consisting of a mixture of tin and bismuth compounds selected from the group consisting of metallic bismuth, bismuth alloys, bismuth compounds, metallic tin, tin alloys and tin compounds for effectively removing copper deposits from a gun barrel, which agent may be mixed into every kind of propellant generating a minimal amount of residue when burned, is provided. The decoppering agent is characterized in that the ratio between the tin and bismuth are between 10-62 % Sn and 90 - 38 % Bi.
We have found that the optimal mixture is 42% SN and 58% Bi.
The melting point the optimal mixture is 13 8 degree C, much lower than the melting point of each of components.
The bismuth and tin either vaporizes or liquefies when the propellant is ignited and either embrittles or dissolves facilitating removal.
Accordingly, it is an object of the invention to provide an essentially lead free decoppering agent that is included in the propellant. It is a feature of the invention that the decoppering agent is apart of the propellant and distributed homogeneously.
It is an advantage of the invention that the decoppering agent is distributed to the barrel and through the barrel with the propellant gas. When the decoppering agent is included in the propellant that also means that the powder gases always contain the same amount of decoppering agent.
The biggest advantage with this invention is that the decoppering agent always is there, it makes the loading easier and cheaper for decopppering.
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