A tail-wind doesn't speed up a bullet. The bullet is slowing from the millisecond that it is no longer under the influence of pressured gases in the barrel.
The effect of a tailwind or headwind is exactly the same as if the bullet had the same speed difference in stationary air.
In other words, a 3,000fps bullet with a 20fps headwind will slow at exactly the same rate as a 3,020fps bullet in stationary air.
The same bullet with a 20fps tailwind will slow at the same rate as a 2,980fps bullet in stationary air.
The bullet has no idea if it is moving or the air is moving around it.
It's also the same if you are in calm air and shooting from a vehicle going 20 fps. If you shoot forward, the vehicle's speed is added to the bullet and if you shoot rearward, the vehicle's speed is subtracted from the bullet. If you are firing at a target that you are chasing and the target is going as fast as your vehicle, it's exactly like shooting into a 20 fps headwind, same drop, same target impact speed.
Come to think of it, when I shoot at a range, I don't even have to think about the fact that the air I'm shooting through is actually traveling about 860 mph. It just seems calm to me because I and the ground under me and the targets I shoot at are also going about 860 mph from the west to east as the earth turns.
All wind is relative. The bullet can't tell the difference between wind caused by its motion and wind caused by the weather.