There is a combination of things in this video that are providing a large dps boost that are bit easier to simulate in the shooting range compared to the rest of the game, namely maintaining 100 stacks of the Striker bonus.
The Striker bonus is very difficult to maintain as it's constantly decaying over time and any missed shot results in the stack rapidly decreasing.
As others have pointed out, this is a lot of different buffs and bonuses that are coming together at once to provide a temporary boost in damage.
/gabelikes