Japan is an island nation that doesn't share a boarder with Mexico in which cartels will happily supply criminals with guns and everyone else will be SOL. Japan also is a homogeneous country with a higher social trust and lower crime rate than the United States. Guns don't really affect crime, the UK is now banning too sharp of knives to try and reduce their crime. The only way to stop crime is to punish criminals and send them away for a long time like the U.S did in the 90's when crime was rapidly increasing.
Mass shootings are a little different. They happen in other places but yes it is a problem in the U.S but it's more to do with social factors than guns. We've always had guns and actually less restrictions on them before and there were rarely mass shooting. Mass shootings got waaaaay more popular after Columbine. Restrictions on guns will not change anything, all that will happens is people will acquire guns illegally ( There's like 380 million guns in the U.S, good luck getting rid of them ) people will also go on to use trucks to run people over, which ISIS did in France to great effect and it killed 80 people which tops our worst mass shooting which was around 50.