If that Reaper was in orbit, things would be very different. There's a reason why Joker avoids going into areas with anti-air weaponry, and weapons are similarly underpowered due to having to devote so much energy to atmospheric operations. This was established back in Revelation before the very first game came out.

Any ship that enters an atmosphere must devote the majority of its power to atmospheric operations, to the point where they can be seriously damaged by light vehicle portable weaponry.

BUT here we have Shepard, standing not 50 yards from the Reaper he's targeting for the entire Quarian fleet - the impact of whose barrage would have vaporized Shepard, Tali, the other squadmate, Legion, and everybody and everything else within five miles of Ground Zero.

The Reapers do it to planets they can't be bothered to harvest. Orbital bombardment obliterates whole cities in fact exploration reveals dead worlds which have been subjected to precisely this form of genocide. In fact the Drill Sergeant in Mass Effect 2 specified that the impact of a 2kg slug from an Everest-class dreadnought releases a third of the energy of the Hiroshima bomb.

The codex has established that just one projectile from a dreadnought-scale mass accelerator hits like a nuclear weapon.

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