Not with a gun but a gimmick. That's how the now-defunct Monarch Construction Corporation of Washington D.C. boosted four and a half million dollars (perhaps six million -- white-collar crime, as you know, is less amenable to strict accounting than stick-'em-ups) a few years ago through a home-improvement swindle operated by a checkered cast of fast-talking and faster-shuffling pitchmen who preyed on blacks who only wanted to get ahead but quickly got so far behind the mortgagee that they had to run by night. The caper -- the fashionable name for this sort of gangsterism today -- might never have come to the point of investigation if the Justice Department hadn't decided that something had to be done about Congressman John Dowdy, a Texas Democrat who was slipped a tidy packet of twenty-five thousand to quash the matter. Carper, an earnest Nader-style advocate who's also done reports on personal safety (To Stay Alive) and the draft (Bitter Greetings), alternately weeps for the duped and wails at the deceivers, who were dealt with rather charitably in court. The moral? Larceny through the gun is so crude/ The fine print is less rude.