Weapons have a multifaceted character. Weapons can, for example, be considered as a method or as an objective itself. It could be cool to hold a weapon, this is as an objective, because it is a passion. As a mean, you can hold a weapon for killing or to defend yourself against others.
As statistics we can see
1984 to 1994: 19 incidents
1994 to 2004 (ban is in effect): 12 incidents
2004 to 2014: 34 incidents
These statistics show the following idea that as an pragmatic effect, indeed, mass shooting has been reduced. I consider the law of arms to be influenced by the tradition of the place where the law is implemented. The number of weapons-related crimes is 25 times higher in the US than in any other developed country.
It's not as simple as you make it sound. The law banned:
Semi-automatic rifles able to accept detachable magazines and two or more of the following:
Folding or telescoping stock
Pistol grip
Bayonet mount
Flash hider or threaded barrel designed to accommodate one
Grenade launcher
The weapons ban did NOTHING of any importance except to be a pain in the a$$ to law abiding people.
So, if you were intent on killing people with a gun you were still able to buy a semi-automatic rifle.
Secondly quite a few of those incidents did not include banned weapons so should not be considered as part of the statistics for or against the ban.