Crisis on Earth-X, Part 3 - S4-E8
Corrected entry: Turns out Oliver was handed an unloaded weapon by evil Quentin. But Oliver's handled enough weapons in his time to know that a gun with no bullets is noticeably lighter than one with, and would have realised the ruse immediately, certainly not then trying to threaten Quentin-X with it.
Correction: It was an unfamiliar weapon from another universe. He wouldn't know its mass.