I did this in the local multiplayer matches, whilst I was waiting on another achievement playing annex. That way I was able to take my time and also avoid boredom. (nobody shooting at me)
To do an active reload:
Click RB to start slider and Click again to stop the slider in the white area - a small vertical stripe the beginning of a larger bar of light grey located under your ammo (upper right of screen).
If you get it right the gun/ammo will flash white.
If it's wrong it'll either just reload or flash red if you are early.
I used the pistol because you can do a reload after one shot and there are 48 bullets, so plenty to use to get this.
Fire one bullet from the pistol and do an active reload.
Basically the same for the next reload only the area to stop the slider in is smaller this time.
For the third active reload I like to fire a few bullets off, which will make the active reload area *big* again. ;) With just one bullet it's a pretty small area now.
And so on.
You can also switch out to another gun and fire off a burst before reloading that weapon, that will also regenerate the size of the active reload area (*the vertical white stripe).
Basically to regenerate the size of the reload area fire more bullets than you reloaded previously. So if you were to do this achievement with a automatic weapon, fire in bursts, starting with a short burst, and making each burst bigger than the last reload.