Damage per Turn: DPT is basically the average damage that is done per hit. Since many weapons have multiple hits, the Total DPT reflects the weapon's actual power. A greater range indicates a more unpredictable weapon, while a lower range indicates a more stable one.
For Example, 5 to 5 damage is stable, while 1 to 10 damage is unpredictable. Both, however, have DPTs of 5.