I use a timer to practice stuff like this at home.
Set the timer for two beeps. At the first beep, you begin the reload, by the time the 2nd beep goes off you should be back on target. The time between the beeps will depend on how good you are. Once you are consistently meeting the timer, shorten the time between beeps.
If you don't have a timer you can still practice this. Also at some point make finding cover while a part of this process. Recognize you are running low, start moving for cover, perform the reload behind cover.
Always remember safety first----no loaded magazines in the room while doing this. Clear the weapon before you ever start.