These mags work well and keep your loadout light and quiet. With Hi-caps you get a loud rattling of bb's inside the reservoir, but with mid-cap mags you are much more quiet. They are very simple in design. You load them with a speed loader from the top, the same place they feed into the rifle from. Two springs, a plastic bb guide and a simple plastic stopper are the only parts internally. Wish one of them didn't come broken. :/
-Quiet on the battlefield which for some is a major plus
-Easy to use
-Great for being more realistic than Hi-caps cause you'll have to reload much more often
-Works well in the Lancer Tactical M4 series. Tested multiple in two different Lancer Tactical models
-Don't hold a lot of bb's compared to hi-caps and some other mid-caps
-Cheap spring stopper can break, making the mag useless
-Just keep in mind they are expandable and are worth the price for what you get
One of mine came broken, then i tried fixing it and it was too far gone.
It's all good though. 1 out of 6 = DUD aren't bad odds.