Yeah within a 50-100 years most of the federal government's ammunition reserves would be spent, the stockpiles that we have right now. 

However in the situation of Fallout, ammo was being produced at unimaginably high rates.

During WW2, the US produced 12 billion rounds of ammunition per year, this is just various small arms calibers, not anything else.

To compare, the US Army purchases 1.8 billions rounds of ammo per year and used 70 million rounds of a year in Iraq per year as well.

If we go by current stock, in our world, then yes the ammunition runs dry within 50-100 years (Seeing that combat wouldn't be that intense, and that it would take a while for everyone to get it all) 

However, even 1 year of ammunition production at WW2 levels, we get enough ammunition, which should last almost [i]200[/i] years if used at the levels they are in Iraq. Which feel more correct to use than WW2's numbers since battles rarely even go beyond a few thousand.

Ammunition, if you end the world during WW2 production levels, is going to be insanely common even after just two or three years of production, and if it was the only ammo around.