Your thumbs hate you. They are still bitter about that time you took a carbine course when it is 30 degrees and sleeting. You made them load all those mags until they were raw. Maybe you owe it to them to take a look at the Box-to-Mag Loader from Three Point Innovations.
This is a very clever device that allows you to load ammo directly from the box to your AR-15 magazines. It works with just about any AR-15 magazine and allows you to load an entire 20 round box of ammo in less than 10 seconds. It should be useful for loading loose ammo as well since you can easily just stack it into the Box-to-Mag Loader.
Sadly, it isn’t available yet, but as you can see from the video, they have working prototypes. They will be available for pre-order soon and you can get a discount by joining their newsletter. Check out the Box-to-Mag Loader on the Three Point Innovations website.