Everyone knew that sales on the Internet have been climbing and will continue to grow. But a new study indicates that Web business growth may be beyond even optimistic projections. \n The study, from Forrester Research Inc. in Cambridge, Mass, predicts that Internet sales revenues will increase from an expected $43 billion in 1998 to $1.3 trillion in 2003. That would mean that more than 9 percent of sales in the U.S. will occur on the Web. \n Who's going to be doing all that selling? According to Forrester, the online industries with the highest sales will be aerospace and defense, petrochemicals, utilities and motor vehicles. \n Here's how it will snowball, the Forrester report said: Trading will start out in a small way. The entry of large firms, including industry leaders, will bring aboard customers and suppliers. Once everyone sees how efficient a way this is of doing business, the variety and volume of sales will boom. \n Forrester predicted the growth of Internet sales early but even its own projections have had to be expanded. In mid-1997, the company said business trade would reach $327 billion by 2002. It abandoned that forecast in favor of the newer and rosier one. \n The revised projections indicate that Forrester believes that the U.S. is further along in its Internet building phase than originally thought and is actually on the threshold of a commercial explosion.