You're walking down a dark street, fear backing up in your throat. Suddenly two shadows slide out of a doorway and move toward you. Knowing you may have to defend yourself, you reach for . . . For what?

Well, in this particular street-crime fantasy, what you reach for depends on which advertisement you believed and which product you bought. You've seen the hype for stun guns, pocket sirens, key-chain Mace canisters and a host of other anti-crime gadgets. Their makers say they'll render you invincible against thugs and rapists. But Massad Ayoob says you may be buying just enough technology to get yourself killed or severely injured.

Ayoob is a 39-year-old former policeman and full-time instructor in armed and unarmed self-defense who teaches both police and civilians. An expert witness in numerous weapons and self-defense cases, Ayoob has been a state and regional combat pistol champion, setting three national records.

"The management of a violent encounter," he says dryly, "is a complex discipline. And when anyone says, gee, here for $15 is a magic button that you push, and it solves the problem -- Lord, if this sounds too good to be true, it probably is."

Let's take the most heavily advertised products and see what a streetwise former cop thinks of them:

STUN GUNS: Legal to possess in Montgomery County, Prince George's County and Northern Virginia but not in the District, these battery-operated devices are slightly larger than most TV remote-control units and pack 60,000 volts of non-lethal electricity. Enough, say manufacturers, to temporarily scramble your assailant's central nervous system. The problem, says Ayoob, is that the average person doesn't know how to properly use a stun gun. "A lot of advertisers imply that if you simply touch your opponent with it for an instant, he's going to turn into a quivering zombie for about 15 minutes. That is not the case."

Ayoob says you must "jam the probes against a good solid muscle group and maintain solid contact for up to three seconds before there's any real dropping effect. But then, if you can hold this violent person absolutely steady for three seconds, you probably have him sufficiently under control that you don't need to stun him."

The real danger of stun guns sold as anti-crime panaceas is that they may instill a dangerously false sense of confidence in the untrained owner. "It's as if I gave you a gun loaded with blanks and said, 'Here, one pull of the trigger will instantly disable an assailant.' The truth is," says Ayoob, "if you stick a guy and touch the button, you're going to have a situation where if he didn't want to hit you before, he definitely wants to hit you now." He thinks stun guns can protect people in certain situations if the user is well trained (no single self-defense technique or device, Ayoob says, is universally appropriate or fail-safe).

Ayoob suggests that would-be stun-gun buyers ask their police departments for help in finding an instructor willing to teach them "pain compliance" techniques that rely on attacking the body's pressure points to subdue individuals. Used in this manner, the legally carried stun gun may be effective. Otherwise, you could wind up being the stunnee instead of the stunner.

SIRENS AND SCREAMERS: These are pocket- or purse-size devices that wail an alarm when you pull a pin. They're supposed to scare off your attacker and call concerned citizens to your aid. Ayoob chuckles. "How many times have you driven through some place and heard a car siren and said, oh hell, some bozo's alarm is going off?" he asks. Similar devices are supposed to "incapacitate your opponent with shock waves of sound." But if you and the bad guy are toe-to-toe, the sound is also going to immobilize you. "It also gives him a compelling reason to punch you in the throat, take the thing out of your hand and step on it," Ayoob says.

HIGH-INTENSITY LIGHTS: Sold locally by one shop as the "Mugger Stopper," this $90 flashlight-size gadget emits "a sudden burst of light" that is supposed to temporarily blind your attacker, giving you time to run. Less expensive devices use multiple flashcubes that pop off when you push a button. But Ayoob says they'll probably disorient you too -- especially if you're in total darkness -- even though you're on the other side of them.

"A bright flash that illuminates a wall behind the attacker will leave a visual imprint on your retina that will remain there for a few seconds, and you'll lose your own sense of direction," says Ayoob. "You may well find yourself running into the wall." In any event, Ayoob doesn't know of any cases where light bursts have been used as self-defense weapons.

TEAR GAS AND MACE: These are not as easy to use as one might think. In Los Angeles, for example, you can legally carry Mace but only if you've taken police-approved training courses in its use (which involve getting Maced yourself so you know its effects and limitations). But Ayoob emphasizes that only trained civilians should carry CN (tear gas) or CS (a choking gas) where legal because their application is particularly tricky. Like stun guns, gas canisters are legal in Montgomery County, Prince George's County and Northern Virginia (except on school grounds) but not in the District.

CS is about 10 times more powerful than CN, but it's slower acting unless you can spray it directly in your assailant's face, says Ayoob. Making a direct hit with gas when you're under attack -- even with the proper training -- isn't easy. "A lot of people don't realize that if they try and apply gas in close quarters, like an elevator or the front seat of their car, they're going to be dosed also," says Ayoob. The other thing you have to remember, he says, is that gas canisters may leak, particularly if you violate federal airline regulations and put one in your luggage and it gets stowed in an unpressurized area.

"When New York reissued Mace to police a few years ago, the patrolmen's union stated that more officers were injured by leaking, defective Mace units than were ever able to use the stuff effectively in the streets," says Ayoob. "Using it isn't as easy as it looks."

KIYOGA: This is a telescopic spring whip widely sold through mail-order catalogues for about $60. Closed, it's the size of a cigar. Press the release, and it snaps open, 17 inches long. The ad copy says "its hornet's nest of piano wire steel springs inflict excruciating agony on your assailant." Ayoob says it can't hurt anybody and demonstrated this in a courtroom in 1979 (New York v. Harold Braunhut) by "slashing my forearm with it as hard as I could. It just turned a little bit pink. By definition, if you can't hurt anyone with it, it's not a weapon." As for "piano wire steel," Ayoob says, "if you can unravel it and strangle him with it, you might do some good."

Ayoob is the author of five books on police firearm and weapons techniques, including The Truth About Self- Protection -- arguably the best book for civilians on protecting themselves at home and on the street. (For a copy, send $4.95 plus $2 shipping to Police Bookshelf, P.O. Box 122, Concord, N.H. 03301). In his books and in conversation, Ayoob emphasizes that gadgets can't automatically or easily protect you against violent assault.

"People want some panacea, some magic button they can press and make the bad things go away," says Ayoob. "When I see people like that I'm reminded of that scene in Being There with Peter Sellers, where for the first time this child of modern technology is actually out on the streets by himself. He's confronted by some bad guys and he takes his little channel changer out of his pocket, presses the button and wishes they'll go away. Well, the technology does exist, but you'll find it in a gun shop, not a Mace store.

"Only lawful, countervailing violence or the solid threat of same is going to deter a violent, potentially armed attacker," he says, "and the gun is the only level of power that will do that for you. But if that's your choice, it comes with an awesome level of responsibility to keep it out of unauthorized hands, to keep it stored safely, to develop enough marksmanship -- not just pure marksmanship but stress shooting -- so you can reliably, with your hands trembling, direct the bullet where it will safely go."

There are no easy answers to self-defense. Says Ayoob, "Welcome to the planet Earth." ::