However, they might prevent someone else from knocking it over, and the bad guys know it. There is no evidence that waiting periods lower crime rates. Such restrictions do, however, prevent people from buying a gun to protect themselves from imminent danger. Nor should adults between 18 and 21 be denied firearms, as if they were uniquely dangerous. Trigger locks would save few lives a minuscule number of children die in gun accidents, less than in many mundane household tragedies but would hinder people from defending themselves.
Individual owners can best balance the one risk against the other.
Licensed dealers already must conduct background checks, including at gun shows. Private individuals need not, but there is no evidence that potential criminals flock to these very public gatherings to consummate illegal deals. There already are 20,000 different laws covering the purchase and use of firearms. Criminals routinely violate one or more of these restrictions to acquire a weapon. They aren't likely to hesitate breaking another one or two.