Careful there, you're making the same mistake I made originally. Current magazines are not attachments,

and as per ChrisL, using the attachment infrastructure for a New Magazine System is not a good idea. See: http://www.ja-galaxy-forum.com/board/ubbthreads.php?ubb=showflat&Number=268676#Post268676

I'm still not sure you could use a straight up attachment system for mags, though. Firstly, you don't want to add another OBJECT structure to a weapon when you don't really need one. Mags are effectively just storage devices so you can simply have the code transfer the rounds from the mag to the gun, then delete the mag during a load operation.

Also, on reflection, a New Magazine System would increase items in items.xml as we would still need the old magazine items for that option to be retained.

There's one problem with those nice calculations of how many items we could save:
Backwards compatibility. As I said, this feature is so OCD that we have to make it optional so that advantage only comes to bear when a mod abandons OMS in favor of NMS. For basic 1.13, we'll only increase the count.