an excuse people make to keep there crutches un-nerfed.

Most common example of bloom is in 3rd person shooters. The reticle expands the longer you hold the trigger and the bullets land at a random point within that circle.

The best way I can explain it is that weapons have an invisible cone of accuracy they fire within in a random trajectory. You can easily see this happen when hip-firing your weapon. Your bullets will land randomly within the reticle, which expands and shrinks with each shot. When ADS, this cone shrinks, allowing you to be more accurate. With non-180 RPM handcannons, however, this cone is unusually large, even when ADS (despite the size of the reticle on the screen). Combined with the high recoil, there's less than a 50% chance of the bullets landing at all. That's why the 180 RPM handcannons are so much better to use on console, compared to PC, where everyone uses the other handcannon archetypes. They don't have that issue.

Bloom is an effect that causes your bullets to not register. This happens when you shoot your weapon at its intended rpm, so you have to pace your shots to make sure they actually register, but they increase time to kill. That’s not only a huge issue, but even when you pace your shot your bullets sometimes just don’t register. That’s why so many handcannons just don’t perform as they should.