Most intelligent cops understand that when politicians talk about "gun control" they're talking about controlling guns available to law-abiding folks. Folks who aren't law-abiding don't give a damn about "gun control," and they always can get guns. In the end, all you do is disarm the good guys, leaving them at the mercy of the bad guys.
If you reduce the total number of weapons and if you have more stringent controls on their storage, registration, sale, and so on, you reduce the avenues for illegally obtaining them (less/harder to steal, harder to "wash" them through a legitimate initial sale, and so on).
Criminals break the law, yes. But if you think that's a valid reason for not having laws, then why have any? Because there is a sliding scale - a certain level of risk that people, including criminals, are willing to take. Make it riskier, fewer take the risk. Make the supply lower and harder to obtain, fewer take the risk.
You can try to argue against that if you like, but you'll be wrong. Criminals take weapons from people who obtained them legally. If there are less legal weapons stored more securely, there are less for them to take. Weather or not you decide to take that option or not is up to the population at large, of course, but the argument that laws about gun control can't stop criminals is a silly one with no basis in fact.