[Legbreaker]

Yes, that's a LOT of weapons in the hands of relatively few.
In my opinion you've hit on a good point with a lot of weapons of the less common calibres running out of ammo in a hurry. Then there's the nukes which would have destroyed weapons along with their owners and irradiated a hell of a lot more. Metal I believe retains dangerous levels of radioactivity for a very long time - therefore a lot of those weapons may still exist, but only the extremely desperate (or stupid) would risk even going near them (let alone into the general area they're lying in).

Another factor would be general decay - owners die from radiation/disease/starvation/exposure/etc somewhere out of sight and the weapon quickly rusts into uselessness.

Additionally, huge numbers of people in the wake of the nukes would be seeking aid from the various refugee camps set up. Perhaps access to them was dependant on giving up any weapons which, unless usable by the police, militia or military, were destroyed - can't have an armed, hungry and starving mob can we?

A percentage of those remaining would have been damaged due to improper or non-existent maintenance. Like most vehicles, a firearm requires oil if it's used regularly (or even just taken out of storage). without it, rust soon develops, rendering the weapon dangerous, inoperable or both.