I got this magazine as a gift from a friend, it is $18 on ehobbyasia and it is officially licensed by magpul pts.
The real steel magazine is actually a bulgarian arsenal 30 round 5.56 magazine, but unlike the airsoft version there is no baseplate. The beta project magazine holds 140 rounds and is translucent, allowing you see how many bbs you have left. It has a nice finish, a nice color and has 5.56x45 marks on the left side.
The magazine has a nice straight form to it so it fits in a vest a lot easier then 7.62 steel magazines. The width of the magazine is the same as the 7.62 one, but is shorter.
The base plate can be replaced with a ranger plate which aids in pulling the magazine out of a vest or rig. To remove the baseplate, depress the button on the bottom and slide it off.
For $18 each, this isn't a very economical choice for a magazine. But for the amount of bbs it holds, plus the extra features, it could be worth it.
Performance test
On the field, this magazine performs well. Strangely, the first 10 bbs only go about 25 feet, but after that the rest feed just fine. So I'm assuming the spring just needs to be broken in. Seeing how many bbs you have left is very handy though.
I give this magazine a 8/10, -1 for the misfeeding on the first 10 rounds, and -1 for the small wobble. Otherwise, its perfect.
More pictures
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base plate