If you're nervous about getting in trouble over any aspect of it, dump the rounds in an amnesty box, then turn the mag in to the first NCO you know who'll take it- there's no particular need to be formal about it, any NCO of yours should be willing to help you out with this, and with this you'll only be handing in a mag, not rounds. Less potential trouble there. I'm not saying bug the first person with chevrons and a leaf who you run into outside NDHQ, but if you go into Cartier Square there should be someone in one of the QMs who will take it, or at least someone who can point you in the right direction. And of course if you prefer, there is always the MP shack at NDHQ.
If you do decide to go to the MPs with it, I'd advise calling first so nobody can plausibly make a fuss of having 'caught' you with it. Some record of your intention to hand it in before anyone else sees it should suffice should someone decide to be stupid about things.
How did this come into your posession anyway, if you don't mind my asking?
Actually, now that I think about it I can conjure a mental image of a half dozen NCOs standing halway across a parade square watching in curiosity as a troop tries to shove an actual mag into the amnesty box. Which begs the question; what's the weirdest thing anyone's ever reported finding in one?