even as the number of guns owned has increased roughly 50% over the past two decades.

Here are the stats through most of the 20th century, through 1990:



Another chart showing 1970-2002, using a different scale (sorry about the non-disinterested source, but the data are National Safety Council and are easily verifiable):



And the accident rate has continued that decline since 2002, and is now IIRC at historic lows.

The thing is, gun accidents are not a linear function of guns owned; they are strongly correlated with inexperience with guns, impulse control and/or substance abuse issues, and (yes) criminal records. The lawful gun culture is, generally speaking, a very safety conscious one, and increases in the ownership rate within that group do not necessarily correlate with gun-exposure rates in the more at-risk populations.