So damage each bullet does is increased or decreased by the percentage given in the BWU, but that doesn't reflect the actual damage difference. If you shoot a gun with base damage increased by 0.04%, a clip of 50 bullets adds that 0.04% onto each bullet. Over the course of the clip, you do (0.04% * 50) = 2% more damage in the same amount of time.
An example with a real gun (Grasp of Malok) which had a 2.97% base damage nerf, while emptying a clip of 30 into an enemy:
0.0297% * 30 = 0.891% less damage
Against a player, assuming two bodyshots of 3 bullets each:
0.0297% * 6 = 0.1782% less damage
To find out how much your gun is affected, simply choose the amount of bullets you want to shoot, and multiply it by the percentage shown on this page The tiny changes are important, because the damage under the scenes is not rounded like what you see visually in-game
I'm not quite sure this is the way to interpret it? You multiply the damage each bullet does by how many rounds in a clip. Not the percentage by the number of bullets. Say you are doing 50 damage output and you are now doing 55 damage output. That's safe to say it is a 10% increase in damage. If you have 20 rounds you don't multiply 10% by 20 to get a 22% increase in damage. You take 50 damage multiply it by 1.1 to get 55 and then multiply that by the number of rounds. So before you did 1000 damage, now you do 1100 damage. Doing it your way implies a 22% increase in damage which would be 1220 damage. A percentage increase is just that, your before and after ratio of damage doesn't changed based on your clip size so that shouldn't even be factored in until you start to talk about how much damage you are doing per second with rate of fire versus what it takes to kill someone.