My opinion, ACC (or I would just call them American consumers because not all are citizens) have increased their demand for ammo which in turn has dried up the supply.
If we are indeed talking about Walmart, I distinctly remember before November 2012 that they were normally flush with 22lr, 223, 9mm etc. After Obama's re-election and Newtown, AC got the scare and panic bought all they ammo they can afford beyond their normal levels.
For example, I used to buy maybe 2boxes of ammo every couple of weeks before the panic--knowing that ammo was abundant and cheap. After November I ramped it up to 3 or more boxes a week, whatever I can afford to buy if there was no limit. Multiply that demand in millions of AC and you get the shortage that we have now.
The American public would have to calm down and slow their demand/purchase of ammo for the manufacturers to finally catch up.