Geese toting M-16s? Well, not quite, but after a successful trial run, gaggles of geese will soon begin guard duty at American military installations in West Germany. Eventually, 900 of the squawking waterfowl, in platoons of six to 40, will take up posts at 30 sites run by the U.S. Army's 32nd Air Defense Command. The idea is not as ludicrous as it may seem. With their acute sense of hearing, geese when startled sound the alarm by hissing, honking loudly and flapping their wings. Indeed, the ancient Romans used geese as guards. The web-footed sentinels are said to have saved Rome by raising a noisy commotion as the Gauls approached the city walls in 390 B.C. What is more, the entire goose patrol will cost the Army about half the annual tab for a single trained guard dog.