Slashdot user MBGMorden has written one of the most concise, insightful summaries of why gun control laws are essentially useless. I’ve reproduced their post here for posterity:
Murder is against the law, and rightfully so, but that’s because the actual act being outlawed is exactly what you want to prevent. Once a murder has occurred grave harm (literally) has already occurred to another party. Same with theft. Same with rape. Same with assault.
See, all those things are directly harmful to another individual. Making laws against them certainly won’t ever stop such crimes from being committed at all, but it will reduce the frequency.
The issue with plastic guns (or gun laws in general) is that the very act of having a gun isn’t harmful. You can do harmful things with it, but just having one doesn’t cause any harm in and of itself. The people that would use those guns to harm another person are already willing to break laws to do so – laws with much stiffer legal consequences.
Think of the number of guns in the US. There are more guns in this country than there are people. The VAST majority of them are never used in a harmful way, and the vast majority of gun owners are law abiding citizens. Passing gun laws affects most of them (because most of them actually follow the laws), but it does nothing for the tiny fraction of them that do not adhere to the law anyways, and those were the ones you really needed to worry about.
Its not that laws in general are useless – merely that laws that exist solely as an attempt to keep someone from breaking another law are useless.
Emphasis mine. That last line sums things up quite nicely.