Sentry Turret always has a 3 round burst, but with AP Ammo each bullet hits twice against armour with the second hit completely ignoring armour DR. It's just a unique ammo consumable power that's being applied to the turret's main gun only.
Flamethrower deals 15 base impact damage at 750 RPM with modifiers against all 3 defense types. Given that it's classified as a proper weapon, it would be getting armour DR applied against it which explains why shields take more damage. This base damage is not increased with power evolutions.
It also applies a Fire DoT effect that lasts for 1 second and deals the damage listed by the ability's tooltip, which isn't affected by the modifiers or armour DR. If the flamethrower hits a target with another Fire DoT already on it, it will refresh the existing one instead and not apply its own Fire DoT.
Rockets will do okay damage, stagger things, and not completely negate the main gun at least.