Kid Icarus: Uprising allows players to fuse weapons. In this process, elements of two old weapons are preserved as they are combined into a new weapon. In general, fusing two weapons of the same power nets a more powerful weapon than either of the two. This can be roughly calculated by a weapon's value, which is 100 or higher.
The results of weapon fusion are shown prior to the actual process, but even before that, it is possible to predict the results of fusions. For example, combining the same types of weapons will result in the same products, even if the exact qualities of the weapons differ. This makes it possible to create specific weapons via fusion.
it is also possible to fuse weapon-gems, although the result have to be paid with hearts.