steel and 240 times higher for stainless steel) higher than nitrocellulose. That propellant in contact with brass is either consumed by turbulence in the barrel or exits the muzzle unignited. Cartridges that are longer but have a shoulder angle less than 35 degrees. (Jameson, patents # 5970879, 6595138, 6550174) or double radiused shoulders per Weatherby do not have a well defined shear line as the shoulder angle is insufficient to trap the propellant in the cartridge case. A substantial portion of the sheared propellant follows the propellant plug down the barrel. In longer cases with mild shoulder angles, all propellant not initially ignited may follow the bullet down the barrel as is the case with straight walled cases. This increases recoil and slows propellant combustion.
As the cartridge becomes fatter and the shoulder angle is made steeper (greater than the 35 degrees claimed by Jamison) the shear line acting at bullet diameter becomes more pronounced between the propellant plug pushing the bullet and the propellant trapped by the shoulder. This sheared surface ignites more quickly than the normal propellant burn rate as previously described. The double burning surface area of the sheared surface adds greatly to the pressure being generated and can be added to the semispherical burning surface originally ignited by the primer to determine peak pressure. Peak pressure is achieved when total area reaches a maximum, early in bullet movement into the barrel. The use of this additional surface area to explain the pressure-time curve in gun cartridges has not previously been postulated or disclosed except in our work.