Penetrating the vent of such small dimensions is one part of the problem but traveling along it quite another. The vents in question are shaped like an inverted L . It's something like this
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Although this is a riser for a clean room with air diffused within the conditioned room from the bottom , air is supplied to the riser from the top. This is as close as I can get in order to pictorially depict an air vent .
You can see the protrusion at the top. Except in case of air vents those protrusions are more pronounced & look like an inverted L from the side view.
Now the Brahmos has to penetrate at an angle perpendicular to the top of the said air vent & then perform a deep dive at right angles to its entry & travel down the length of the vent before exploding at the desired depth.
How's it possible ?! I've been trying to rack my brains ever since I first came across this bit of news. The air vent being 45 & 45 cms doesn't lend itself to the kind of contortions the Brahmos would've to perform to enter the vent & then take a path perpendicular to its entry to travel the length of the air vent before arriving at the designated point where it's meant to explode.
There's something we aren't getting right here assuming the Brahmos was indeed used to blow up the underground C&C at Nur Khan Bholari & other ABs .
I'm of the impression the first strike to blow up the vents & lay it bare must have been the task of some other munition perhaps an LGB before the Brahmos was deployed to enter into the now prised open vent coz these ventilation shafts follow the principle of 2:1 where the vent we see ( the air duct essentially ) is the supply air side where atmospheric air is drawn in & supplied within the conditioned room & the surrounding space acts as the return air which is twice the size of the supply air duct from where the air within the room is expelled out to the atmosphere thus ensuring proper ventilation thru supply of fresh air within the conditioned space.