I really feel the need to question that assertion, at least with respect to handguns. The most popular handgun currently seems to be the Glock. Glock is made in Austria.
Of the remaing brands, Taurus handguns are made in Brazil, Charles Daly in Turkey, Rock River in the Philipines, Para Ordnance in Canada, and Springfield Armory's XD series in Croatia. I've taught CCW classes in southern MIssouri and worked for a brief time in a pawn shop/gun store. The above named brands, along with Beretta and Walther are by far the most commonly seen and purchased at least in my part of the country. S&W handguns are starting to gain some popularity with the new M&P pistols, Ruger seems to be generally unpopular with the exception of their Mini-14 rifle, and Colts are considered collectors' items. In short, I really think that it's questionable to claim that most guns are made in America anymore.