This looks a lot like the mag that comes with the A&K M249. If it is, or similar, be warned, heres a review on this mag.
This mag is great, but only for a little while. The batteries in the mag slip out constantly, which demands a piece of tape to hold them in place. Now the switch allows for auto winding, which is supposedly supposed to sense when the bb's at the ready are low, and then wind. Mine (which i bought slightly used) just winds forever. I mean this is like dry running a motor. it doesn't take a genius to understand that leaving it on will ruin the mags auto wind motor, as well as run through the batteries far faster.
So basically you are forced to hold onto the switch to turn it on only when needed. If you plan on holding down the trigger for 10 straight minutes, then sure leave it on. But stopping at all hurts the motor, and is a waste of batteries. So like i said you hold the switch, which is at the bottom of the mag. Imagine holding this gun with a foregrip. Now imagine holding it with a foregrip, but the foregrip is actually a 6x8x4 inch box, with no texture/grip. That's what you are forced to do. It's very awkward, uncomfortable, unmanagable. Hard to aim when you are holding it like that. When prone it's hard to get a hand between the mag and the dirt. etc.
Typically this mag feeds best when completely full. Once 30% full or less, it starts to skip feeds, jams, etc. This is typically fixed by shaking the mag/gun (which loosens the bb's allowing them to fall into the auto wind slot), and or pulling on the flexible spring that the bb's run from the mag to the gun through.
There are many mags that are 100$ or more. While that seems like a lot, i can't stress how much i recommend those. If you are serious about box mags and you buy this, you will spend 50 on this, then 100+ on the mag to replace this POS. Skip the 50 and go for the real deal.
Good fresh out of the box,
Good when full
Horrible auto feed controls
Horrible auto feed location for controls
Jams, more so when nearing empty
bit of a pain to attach/detach (though this is a small problem)