How to skip entire building in Water Hazard

On the chapter Water Hazard, in the map d1_trainstation_07, you can actually skip the building you have to go into to open the big gate.

1. walk to where you can see the explosive barrel that is used to blow open the gate
2. use the suit zoom to get a good view of the barrel (optional)

3. shoot it, preferably with the pistol as it's more accurate

Once you've successfully hit the barrel a few times it should blow up, plunging the metal thing into the gate, destroying it. All without having to go into the building.