By making it more difficult or expensive for me, as a law abiding citizen to purchase weapons and ammunition that are legal for me to own now. That's one example.
I own weapons and ammo that would look pretty bad if they were layed out on the floor for CNN, but they've broken no laws to date. Now the law changes that restricts the size of magazines or the number of rounds of ammo I can purchase for my own legal use.
Now, for your point, Imagine it's currently legal for you to buy alcohol at age 21. A 21 year old legally buys alcohol and drives his car into a movie theater killing many people. Now, the government should make the legal drinking age 25. Maybe that doesn't work to explain what I mean.
Maybe you should give an example of a law that only affects criminals that currently doesn't exist on the books.
"Silence is Golden. Duct Tape is silver"