Take the AR-15, the auto-loading rifle that Adam Lanza used at Sandy Hook. The AR-15 is the civilian equivalent of the military M-16 assault rifle. It's one of the most popular assault-style rifles on the market today.
In 2009, in a declaration made as part of the court case Heller vs. District of Columbia, which challenged D.C.'s assault weapons ban, NRA research coordinator Mark Overstreet reported that, from 1986 to 2007, at least 1,626,525 AR-15-style semi-automatic rifles were produced and not exported from the United States. Overstreet suggested that you could use trends in NICS background checks to project future sales of AR-15-style rifles. As of Nov. 30, 2012, the total number of NICS background checks increased by 50.4 percent since the end of 2007. If the number of AR-15 rifles increased similarly, then that means there are at least 2,446,294 AR-15 rifles currently available in the United States.
That "at least" is an important caveat. These data only include firearms manufactured in the United States. In his declaration, Overstreet notes that, since 1986, "U.S.-made firearms have accounted for roughly three-fourths of all new firearms available on the commercial market in the United States." So if you increase the above number to account for foreign-made, AR-15-style rifles, you get 3,261,725 total rifles.
More caveats: Overstreet derived his numbers by examining the sales figures of companies that only produced AR-15-style rifles. He didn't include sales data from America's three largest gunmakers — Remington, Smith & Wesson and Sturm-Ruger — because these three produce multiple lines of rifles, and he couldn't break down the data. (Remington is now a wholly owned subsidiary of the Freedom Group.)
Let's try estimating those figures ourselves.
According to Sturm-Ruger's 2011 annual report, sales of rifles accounted for $83.4 million in revenue that year out of $324.2 million in total net firearms sales — about 26 percent of revenue. Sturm-Ruger produced 1,114,700 firearms in 2011. Assuming that every gun cost the same amount — this is certainly incorrect, but we're just making a back-of-the-envelope calculation here — 26 percent of 1,114,700 is roughly 290,000 rifles. According to Overstreet, AR-15s accounted for 14.4 percent of rifles produced in 2007. If that statistic remains true, then Sturm-Ruger produced close to 42,000 AR-15-style rifles in 2011. Walk that number back through the years, and extrapolate it out to the other two manufacturers, and you're possibly looking at anywhere from 500,000 to 700,000 more AR-15-style rifles. (Again, I must caution that there is a potentially huge margin of error in these calculations.)