But let's consider why and how Japan was so successful on getting rid of the gun crimes. At least not Japanese by themselves in 100% It were the WWII victors who wanted their people completely disarmed so they removed all firearms from the civilian sector and army was reduced to the minimum.
Stop. You don't know a thing about Japanese history at all, and your arguments fall apart right here. This didn't happen, mainly because it wasn't necessary.
Japan's history with arms control goes back at least to the Tokugawa Shogunate, if not earlier, and the Sword Hunt that sought to prevent non-samurai from owning blades longer than the proscribed length. They've been doing gun control-like measures since before they had guns. As such they've been wildly successful in lowering the amount of violence (the current gang war is equally notable for the fact that few people are getting stabbed, again because of regulation
on the ownership of knives instituted after the mass-stabbing incidents of the '70s).
For an alternate answer, we could consider the low rate of firearms crime in the UK, which has been regulating the ownership of personal firearms at least since WW1 as a response to the IRA. How would you opt to explain that?