In 2000 the U.S. Department of the Interior reported that excise taxes on ammunition generated $68 million, whereas in 2012 that figure was $207 million. With inflation taken into account, that’s approximately a 129 percent increase in 12 years. A lot of that growth has taken place in the past few years. Between 2007 and 2012 excise tax money generated from ammunition sales almost doubled from $108 to $207 million. Tax dollars from ammunition sales were stable from the mid-1990s through 2006, but then started to climb fast as gun sales began surging.
To understand what $207 million represents, it’s helpful to know that in 2012 the NSSF estimated the size of the consumer rimfire, center-fire and shotshell market at about 9.5 billion shells and cartridges. That includes U.S. production in addition to imports minus exports. Last October the NSSF predicted there would be more than 10 billion cartridges and shells made for the American consumer market in 2013 as manufacturers attempt to keep pace with consumer demand.
Obviously this massive increase in demand has made it difficult for ammunition makers to keep up. Can you imagine what would happen if the demand for your other favorite products doubled in five years? Wouldn’t they likely be more expensive and harder to find? Also, ammunition production can be difficult to increase quickly because it takes investment in expensive machinery and additional personnel to increase production. Making more ammunition also requires companies to purchase more raw materials in a competitive and international marketplace. Manufacturers also must worry about overinvesting in a market bubble and thereby getting caught overextended in coming months or years.
http://www.americanrifleman.org/articles/why-weve-had-ammunition-shortage
These are valid concerns even in areas that are relatively unregulated such as raw materials (aluminum, copper, iron and so on), where a significant number of companies are losing billions due to overcapacity. Given that state-by-state or federal gun or ammo bans could crater demand very rapidly, it's understandable that ammo-makers are being cautious.