OK, what is the difference?
Clips:
Mags:
photos by Oleg Volk
From the NFA website:
Magazine vs Clip
A magazine is an all encompassing metal or plastic ammunition feeding device incorporated in or designed to be attached to a repeating or autoloading firearm. The law restricts the capacity of magazines used in autoloading centre-fire rifles and handguns.
A clip is a metal device designed to hold or store ammunition in an easily portable package. This saves having pockets full of loose cartridges. With few exceptions (M1 Garand), clips are not designed to be inserted into a firearm to feed ammunition while the firearm is being shot. There are no restrictions to the capacity of ammunition clips because they are not in the firearm while it is being shot.