Patent Publication Number: US-2005115452-A1

Title: Decoppering agent

Description:
This application is a continuation of PCT/SE03/00028, which designated the United States filed Jan. 13, 2003, and which in turn claims priority from Swedish application 0200366-3 filed Feb. 8, 2002, the entire disclosures of which are incorporated herein by reference. 
    
    
     TECHICAL FEILD  
      This disclosure relates to an addition to every kind of propellant effective for removing copper deposits from the inside surfaces of gun barrels. More particularly, a composite addition that has a pulverized decoppering agent mixed into the propellant.  
     BACKGROUND  
      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, and thus 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 out 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 chromium facing. The gun barrel is harder than the rotating band of jacket, which is typically copper or 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 lead-free primers. Before, the primer contained a small amount of lead, which 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 or 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 fractured debris is swept from the muzzle of the gun with the propellant gases.  
      A second theory as to why 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, and the copper bearing the lead solution is then 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 lead free decoppering agent.  
      Among the lead free decoppering agents that have been proposed are bismuth, bismuth subcarbonate (BiO) 2 CO 3 , tin, and tin alloys. But bismuth compounds are very brittle and even metallic bismuth cannot be rolled into a thin foil like lead. Alloys of bismuth metal with other metals can be rolled into a foil, but the alloys are very expensive and less effective as a decoppering agent. There are also solutions where one uses specially made pellets of bismuth in a nitrocellulose binder, which are then added to the propellant charge.  
      There remains, 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 than having to add a specially made decoppering additive to the charge. 
    
    
     DESCRIPTION  
      There is provided a decoppering agent that is incorporated into the propellant to remove copper from the rifling of the internal bore of a gun barrel. The decoppering agent consists essentially of a lead-free pulverized additive mixed together with the other propellant compounds. We have found that a mixture of tin and bismuth is a very good decoppering agent. Particularly, the ratio between tin and bismuth is between 10-62% Sn and 90-38% Bi, and especially in the ratio 42% SN/58% Bi, the melting point for this combination is 138 degrees C. This melting point is much lower than each of the components. The decoppering agent can contain all kinds of suitable tin and bismuth compounds. Some suitable compounds are selected from the group consisting of metallic bismuth, bismuth alloys, bismuth compounds, metallic tin, tin alloys and tin compounds. The bismuth and tin either vaporizes or liquifies when the propellant is ignited and either embrittles or dissolves the copper deposits facilitating removal.  
      Accordingly, this disclosure makes possible an essentially lead free decoppering agent that is included in the propellant. In a particular embodiment, the decoppering agent is a part of the propellant and is distributed homogenously throughout the propellant.  
      It is an advantage that the decoppering agent is distributed to the barrel and trough the barrel with the propellant gases. Including the decoppering agent in the propellant also means that the powder gases always contain the same amount of decoppering agent.  
      The biggest advantage is that the decoppering agent always is there, thus making the loading easier and cheaper. No need exists for extra containers with especially made substances for decoppering.