Sure, there may be enough ammo in the US to fight a 1000 year war, but is there enough ammo in your 100 mile radius to last more than 10-15 years?
Why would you stay in a place after you exhausted the resources? Supply will of course vary by location, and use would be dictated by your location as well... And at least for me yeah I would place damn good odds on there being enough ammo in my area to last well beyond my life... But, if I'm in daily or even weekly gun battle gobbling up ammo (time to vacate and move) I'm confident that my chance of dodging bullet puts me at a higher risk then running out of bullets in most cases... And honestly I'm 100% positive that within 10-15 years if my survival depended on it I would have found a re-loader and supplies to reload to extend my life line or moved on in search of supplies... The knowledge and and raw materials simply wouldn't vanish into thin air, and if my life was depending upon it I would find them...
99.999999% of you Internet Cowboys wouldn't have the guts to pull the trigger on another human being.
If that was true the homicide rate in the US would be zero each year...
But the fact is that happens every day in this country, there are PLENTY of people that already DO pull the trigger on another human in self defense (or just because they are annoyed, having a bad day, want the $10 in your wallet, or the keys to your car) every single day in this country, it's far from a 1 out of 1,000,000,000 statistic... Escalate that to a life or death decision in a post apocalyptic world where you know you are the last line of hope for yourself, no 911 call, police or hospital to run to, and I'm willing to bet the likelihood of people pulling the trigger jumps exponentially...
Especially if you run out of ammo for them.
You can still use a pistol or rifle as a melee weapon bullets or no bullets... And melee weapons are hardly void of failure as well, what if your melee cracks in half, is sure is going to reduce it's effectiveness quite a bit, no?