It is almost certain that an all out ban on guns would cause a decrease in violent crime in the short run, but that is not what the congress is proposing. The current gun control provisions of the Juvenile Justice Bill are virtually insignificant to the rate of violent crime because they are targeted at very specific aspects of gun acquisition and gun control that mostly affect law-abiding citizens. Therefore the short run changes in violent crime due to the current gun control provisions are uncertain. Consider the scenario that the current gun control legislation would reduce violent crime in the short run and that this would yield positive social benefit. This is not enough evidence that the bill should become law.
Some policies have positive social effects yet lack coherent long-term positive social benefits.
It is quite possible to have a policy with short run benefits that could be detrimental in long run. Congress may be engaged in this social benefit at any cost thinking as it prepares to return the projected budget surplus to the American people. Congress has passed a tax cut proposal based upon a ten-year projection of the budget surplus. In the short run Americans will have more money. But the effect over ten-years is uncertain.