That was a very lovely description btw.
Bloom is how much your reticule will, wait for it, Bloom(or expand) while firing your weapon.
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Bloom is more commonly known as the "cone of fire". Here's my cool art of a barrel and a cone:
=<
Picture the point of a cone extending away from the barrel of your weapon with the larger end of the cone expanding the further it gets from the gun. While influenced by your aim, your bullets can land anywhere inside the cone. On many weapons the cone will expand the longer the trigger is held down, on others it will narrow.
As others have pointed out, that's different from recoil which will typically drag your aim vertically as you fire.
There is some info here that is correct and wrong...
Recoil as others have said is the how the crosshair moves. There are two types of recoil Horizontal and Vertical. Vertical is the more common of the recoil.
Bloom is the change of crosshair during sustained fire. So if your crosshair looks like pinpoint and as you hold sustained fire it goes from being a pinpoint to being open. Say about an 1/4 of a inch on screen.
Now Cone of Fire is what makes your weapon accurate at different ranges. Ever wonder why a LMG though with a small pinpoint reticule still misses it's target at say 75 meters? It's because of CoF. Imagine an Ice Cream Cone on the tip of your barrel. With the smaller end being at your gun. The longer the distance from the gun the larger this cone is. If this cone completely surrounds your targeted distance than your bullets have a chance to hit anywhere in that cone.
Now Bloom and CoF go hand in hand with accuracy at ranges. The larger the start of your cone the larger the cone will be at the end.
bloom is basically the longer you hold down the trigger on your gun the bigger your crosshair spreads apart
stand ontop of target and shoot.
tada bloom negated.
but srsly when using automatic weapons fire in short burst. click 0.5 sec release click 0.5 sec release and so on.
bangbangbang <-->bangbangbang<-->bangbangbang<-->bangbangbang = high sustained fire at twice the accuracy of
bangbangbangbangbangbangbagbangbangbangbangbangban gbang and with recoil your aim
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-----------------------/------------------------------ targets head
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........................../--------------------------------targets body and main target for sustained fire
Bloom: The more you fire, the worse your accuracy gets per shot, until you stop firing briefly and let it settle.
Accuracy: The ability your weapon has to put bullets on target with your exact target dot / crosshair. This is affected by bloom.
Recoil: The tendancy for your weapon to kick upwards when firing (most typically associated with fully automatic weapons, but affects all semi-auto weapons to some extent).