Doug Mataconis writes about California's drop in gun-related injuries over the past decade despite the fact that gun sales have nearly doubled during that time:
There are several reasons that gun related deaths an injuries have fallen, of course, not the least of them being a nationwide drop in violent crime....Nonetheless, the fact that a significant increase in gun ownership has not led to an increase in gun injuries or deaths would seem to undercut one of the primary arguments of advocates of gun control. Contrary to their assertions, it would appear that allowing law abiding Americans to own guns doesn’t lead to an increase in violence after all.
That just doesn't follow. As Doug mentions, violent crime has fallen dramatically over the past couple of decades. Without controlling for that drop, you simply can't draw any conclusions at all about the role that guns play.
As an analogy, traffic deaths in California have declined by a third since 2002 despite the fact that the number of registered vehicles has gone up by about a third. Does this mean that more cars and trucks don't lead to an increase in vehicle deaths? Of course not. Fatalities are down because of airbags, antilock brakes, higher seat belt use, improved ER technology, and so forth. If you controlled for all that, you'd likely find that more vehicles do indeed lead to more highway fatalities. But you'd have to do the math to know for sure. Until then, you really can't conclude anything at all.