Abstract: Simple models for the internal, external and terminal ballistics of a tank gun firing a long rod penetrator are described and used to determine the penetration that can be achieved for a typical system. The models also show the effect on penetration of modifications to the penetrator geometry, charge parameters and barrel length. The results show that there are many interdependencies, and that an apparently simple modification to improve penetration may be accompanied by unacceptable consequences elsewhere, particularly high breech pressure and sabot stresses, and an inappropriate all-burnt position. It is also shown that these effects can only be mitigated by further modifications which tend to negate the apparent improvement. The most promising improvement in penetration is based on more energetic propellant and is about 12%.