A homemade pressure-cooker bomb that failed to detonate on West 27th Street was designed for “pure devastation,” an NYPD detective who defused the device told jurors at the trial of accused Chelsea bomber Ahmad Rahimi.

“Picture it like a thousand guns going off at the same time,” Marine-turned-cop Jason Hallik told the Manhattan federal jury of the bomb — one of two the feds say Rahimi planted.

Hallik was one of two officers who disassembled the West 27th Street bomb after a similar bomb on West 23rd Street injured 31 people.

Earlier, prosecutors wheeled out a mangled dumpster that had been blown across the street when the 23rd Street bomb went off.