[rdouglass]

From my experience, it seems to be a ratio of troops to population, not the weapons power of said troops. In fact, some threads discuss using 'police' troops - empty troop 'shells' with just a cockpit - as being quite effective.

Also Urban Pacification Fac's and/or Racial equivalents (Fate Shrine, Temporal Vac, etc.) are almost required to keep unruly populations in line, especially after your empire reaches a certain size (I don't know what size that actually is however...)