You can fit 16 rounds in it! It didn't fit my Helios but, it fits like a glove in my Apollo. Only complaint would be when I have 15 or more rounds in it, it tends to jam when feeding but, other than that it's an amazing upgrade for the Apollo.
I have seen tons of reviews all saying it doesn't stay in the mag well and to my surprise it holds better than the nerf rival mag. And no i wasn't paid to say this, you are probably thinking oh its fake because he said it was good but no this is my opinion and i stand by the fact that its very good.
They only feed in the Apollo and Helios but don't engage the mag release so they just fall out, they also don't feed all that well with knockoff ammo.
I bought two of these for use in a Rival Helios. SHOULD have worked fine (just like a Apollo right?), but for whatever reason, they wont engage the magazine retention mechanism on the Helios and just fall out of the blaster. As a result, OOD removed that model from their "compatible" blasters list. MIGHT have just been my Helios, but hard to say. OOD was most gracious about it and refunded me since they would not work; thanks Luke!. Hard to comment on how well they work overall since they would not seat in my Helios, although if I held it in place, it fed and shot fine. So I expect they work great in a compatible baster, like the OOD Jupiter. Another quirk to be aware of: because the base of them is "squared off," they also won't work with Tarik's MOLLE-compatible magazine holders.