I got these a couple weeks ago, and since I received them, I have been nothing but impressed. The build quality is fantastic on each and every one of these, even though they are plastic. I had a set of King Arms midcaps for my MP-5, and they were exactly the same; solid. I don't know what King Arms did to make these so strong while keeping the weight down, but it works really well. Once fully loaded, these magazines never mis-feed, or double feed. In fact, I loaded these all the way up, and they can dump their entire load of BB's in just under a second, so unless you have a gun that shoots over 100 rps, you should be fine with these.
Even though these are advertised that they can hold 120 rounds, I find they work best with 100 rounds only.
Something else about these: On the bottom there is a twist off cap where you can access the spring tube so you can remove the spring to lube the tube, or take the spring out to stretch it. Also, in the spring tube there is a plastic spacer at the end that you can remove to increase the capacity of these magazines. If you decide to do this however, I would recommend stretching the springs so that they can fill the extra 3" of space that the spacer fills.
Overall, I would recommend these magazines to anyone who is interested in a solid set of midcaps for their M4, or M16 rifle. Since they are so cheap as well, I do not hesitate when a team mate asks if they can borrow a magazine from me during battle, even if i have to throw it to them.
They come in a 10 pack
Spring can be removed easily
Perfect fit in my G4-A1 Magwell
Easy to lubricate if required
Don't quite fit 120 rounds
Rating: [5 of 5 Stars!]