The men at the AVCO Corp. briefing booth can be very friendly. They have a full-scale mock-up there of the MIRV ballistic missile system payload, a nosecone about 10 feet high that is filled with black dart-like nuclear warheads each the size of a man.

The MIRV is one of the most popular of the 60-plus weapons and hardware displays at the Air Force Association convention, a trade show of airborne war power that has brought 5,000 visitors, half of them in uniform, to the Sheraton Washington Hotel this week.

"Of course it's impressive because of its size," said John M. Gilmore, "but here's a nice little weapon you don't want to overlook." He drew his visitor over to a small gray robot with three springy antennae, roughly the size of a squashed breadbox.

"This is our Extended Range Anti-Armor Munition," Gilmore said. "Drops by parachute and waits on the ground until an enemy tank comes by." The ERAM blows one hole in one tank with one shaped projectile, passing through armor plate as if it were chocolate mousse.

As Gilmore was explaining this in a corner of the AVCO booth, the Red Chinese walked in.

Francis R. Shuttes, another briefing attendant, showed them the MIRV. Zhang Wenyi, the air attache of the People's Republic of China, seemed very interested. Then Shuttes showed Zhang the ERAM, and answered a few questions. The Chinese contingent of three looked at the armor plate with the hole in it, and seemed very interested. Then they were off to their next stop -- perhaps the MX rocket system briefing, or a look at the mock-up of the cruise missiles nearby.

But first, a question: And how did you enjoy our MIRV, Mr. Zhang?

"We just came to tour the place, to get a general idea," he replied through an interpreter, taking a reporter's question in stride. "We just arrived. We're interested in new hardware. We'r just leaving." And they did, merging back into the sea of Air Force blue.

Shuttes had a large grin on his face.

"Last year they came and took pictures, with one guy holding up a ball point pen as a size reference," he said. "This year, we have a trick."

He took out a small card and pasted it on the life-size warhead that dominated the booth. The card read: "Quarter-scale model." "We already tried the card on a Russian scout at the Paris air show," Shuttes explained. "Boy did his eyes get wide." 'Apocalypse Now' Fireballs

Everybody's eyes get big. This is the nuclear shopping mart, all right, the arms bazaar against which picket lines always form, the maintenance shop of the balance of terror. Yet it feels like the boat show at the New York Coliseum. Nobody walking the new carpets under the bombs and rockets, shopping-bag full of literature under his arms, seems worried.

Over at the General Electric booth, a tape machine projected an unending advertisement for the GAU-12/U "Equalizer," a lightweight 25-mm Gatling gun with numerous applications in air-to-air, air-to-ground or ground-to-air roles.

On the tape, a tank rolled into place next to an armored personnel carrier. Both vehicles had large red stars on them. The GAU-12/U commenced firing, sending a swarm of tracers into the vehicles, which soon became "Apocalypse Now" fireballs. "We're using a depleted uranium core in these rounds," the GE briefer said. "For their density and incendiary effect."

At the Fairchild booth, Eleonore Raabe answered questions. "Just don't ask me anything about airplanes," she said. Raabe has been a hostess at this convention before. "I was here the year the demonstrators got in," she said."They threw chicken blood all over the place. Security is tighter this time."

Fairchild builds the A-10 Thunderbolt, a strange-looking jet fighter designed to provide "effective anti-armor support in a high-threat air defense environment." Behind Raabe, a movie showed the Thunderbolt in action.It has real wings, not the paring-knife blades of other jets, and it can bank and turn on a dime. It can scream low into valleys, then scream back again like a crop-duster. It looks extraordinarily mean and specialized. b

"Survivability!" the sound track promised, warning that "the density of ground defenses among Warsaw Pact nations tells us that NATO aircraft will take a beating." Not the Thunderbolt, however, which can "turn away from the lethal envelope of enemy fire." It also turns tanks into "Apocalypse Now" fireballs. "Ruggedness, survivability, massive lethal firepower!" Galactica Hard Hat

"Pull the helmet on your head," said Rick Mostrom of Honeywell Inc. in the tones of a dentist about to administer painless extraction. The helmet, which looks like something out of the prop room of "Battlestar Galactica," is what a fighter pilot would wear if his plane were equipped with LAD -- Laser Acquisition Device. Behind the visor, the pilot sees a landscape as usual. But he is seeing with laser power, and soon a pinpointed target appears. A tank, perhaps. Then the "Apocalypse Now" fireball. Thank you for the demonstration.

Honeywell is hoping to get sole-source production of the LAD by Feb. 1. "We expect organic capability by June of 1982," Mostrom said. That is if a competing system doesn't get the go-ahead first.

Boeing's MX display is right by the door as you go in. The MX, of course, is the "shell game" ICBM system planned for installation in the desert along the Nevada-Utah border. The price is more or less $33 billion, for which you get numerous clusters of missiles spread out over 30,000 square miles.

How it works is demonstrated by a model train set, known at the Sheraton Washington as the "Lionel MX System." A car riding on tracks delivers a missle to a silo. The shell game is, which silo? Each missle could be in any one of 23. When it is time to end the game, the real missle literally stands up. But the MX system hasn't got much sex appeal. It's a deterrent, not an active, usable piece of hardware. The Air Force colonels looked at it, made small talk and walked away.

The cruise missle is easier to warm up to. It looks like a 30-foot version of a very sleek model plane, and it has neat little wings and an air scoop that looks like it was lifted from an Aa fuel dragster. It may not have a pilot -- who wants to pilot a flying bomb? -- but at least it flies . The people at Williams Research Corp. are very proud of the small turbofans which power it, and which they produce.

Of course, with so many big-time companies conpeting for an audience, if you don't happen to have a cruise missle, you have to do something. Three conpanies chose magic.

Paul Gertner, performing at the Bendix booth: "Let this simple deck of cards represent the APS 133 Color All-Weather Radar system," he told a crowd of 40 people. " . . . I ask you to remember this card, and also the Bendix Cockpit Display System for Swept Wing Aircraft."

The magician at Lear-Siegler's Astronics Division booth ("flight control and avionics") was running out of time. His tour group of officers was scheduled to be somewhere else very soon. "Two minutes?" he said incredulously. "We gotta go fast."

One member of his audience wore a chrome-plate helmet with a light bulb on top. Another "assistant" had an aircraft steering wheel in his lap. Somehow or other -- maybe this was a little speeded up -- the steering wheel made the light bulb light up, and this signified the corporation's data transfer system. l"How much time do I have now?" the magician asked. Dumb Bombs, Smart Bombs

How to make and "Apocalypse Now" gireball out of a tank, continued:

"Some submunitions are smart, and some submunitions are dumb," explained Jack Reinhardt at the Brunswick Corp. booth. The context, of course, is LAD -- Low Altitude Dispenser for armor attack with submunitions. Brunswick's LAD is a 12-foot-long bomb-like projectile launched by a fighter. What it does after that is drop 48 little bombettes. Smart bombettes head right for enemy tanks, converting them into Apocalypse Now fireballs. Dumb bombettes need a little more luck.

(Brunswick's LAD should not be confused with the Honeywell's Lad -- which is another acronym entirely).

"The good thing about the LAD is that the F-16 pilot can come in real low," Reinhardt said. "He can penetrate the battle area at 100 feet, under radar, launch the LAD and then get out of there." The LAD itself, left behind climbs much higher before dispencing its bombettes. All this for only $21,000 each, submunitions not included.

"You don't want to overfly a target area anymore," added another Brunswick Corp. briefer. "They got stuff coming up at you nowadays. Do not overfly your target."

Over in a corner, the Air Force has errected a diorama depicting its own PLSS system (Precision Location Srrike System). It looks like something out of a Smithsonian Instituttion exhibition on mountain valley farming, with tiny cows and a river and a bridge. On one side of the bridge, however, are Russians. Tiny trucks and rocket launchers with tiny Russian Cyrillic letters, and tiny . . . tanks. PLSS is very good against radar-emitting targets, and also quite effective against tanks.

All over the exhibition hall of the Sheraton Washington today, tanks, especially, will continue to turn into "Apocalypse Now" fireballs. In movies, tapes and diagrams they will, in various ways, be shot full of holes, vaporized, caused to fly apart and be rendered useless. Most of the tanks suffering obliteration have a red star on them.

An Air Force colonel litened to this observation while gazing distractedly into the diorama. Conventions, he said, made his feet hurt. As for the tanks:

"You will notice that the Warsaw Pact countries have a lot of tanks. Many more tanks than we have. The idea is, if we fight, they should lose those tanks,"