It sounds too simple to demonstrate on video, but I've learned that filling up those big sticks isn't always so obvious or easy. These videos show a few tricks to make it easier and also to ensure the rounds are stacked up correctly.
Note! On the 21 Round mag, this is with a brand new spring and it will only load to 20. Use it as a 20 round mag for a while until the spring takes enough set to allow it to load to 21. In the video I demonstrate how hard it is to get the last one in. Until the spring is ready you shouldn't force the 21st round in there. Shoot it as a 20 round mag for a while ! (At the end of the video I say shoot it as a 21, but I meant to say 20..)