D is internal diameter of the brass case
dO is diameter of the outer shear line
dI is diameter of the inner shear line
lO is length of outer shear line
lOC is compression factor for the propellant at first bullet movement
lI is length of inner shear line. This term disappears when bullet movement exceeds inner shear line length.
lIC is compression factor for propellant at inner shear line
mb is bullet movement at time t