THE YOUNG man staggers, hand clutching his chest in mortal agony, drops to one knee gasping and falls, arms outflung on the grass. Over his head, his comrades exchange a volley of shots, almost obscured by smoke from the guns. Beyond the fence a second group is scrambling up the hill bearing a faded and torn flag. In the distance somewhere a bugle is playing.

It's the newest fad in these parts and north to New England - vintage American battle reenactments - and it's caught on with an eager, loyal band of volunteers ranging from shoe clerks to museum curators. Young Professionals in their mid-30s join up for a recreational week youngsters during the Civil War centennial, love the excitement of recreating that brother-against-brother war.

Battle reenactment combines the best aspects of strenuous exercise, that important sense of belonging, and a closer look at history. With fast footwork, fans can indulge their hobby every weekend through summer and fall. Join up or be a spectator.

The South never really got over the Civil War and they're replaying it these days from Richmond to Frederick - especially the battles where the Yanks got beat. The generation that tuned out the Vietnam war loves the earlier ones.

"A lot of the guys in my unit were really against Vietnam," says Larry Bopp, a Baltimore-area teacher who is a captain in the 4th North Carolina Civil War unit. "It was during that war that they got interested in the Civil. The issues were so cloudy for Vietnam. They're much clearer in the Civil and guys identify.I had two ancestors who fought for the Confederacy and one who was a Union drummer boy."

"The Civl War is a non-threatening war," says Dr. Lynn Sims of the Richmond Bicentennial Commission who holds a doctorate in U.S. military history," and These aren't real bullets."

They look real enough. They seem to kill. Smoke fills the air with what appears to be the smell of war. The aim is so careful, it must be for real. More than one spectator has found himself caught in the line of fire and quickly hunkered down uneasily behind a fence.

The war buffs are up on their history and anxious to make it all as much the way it was as possible.

"I don't think there's a farb here," Sgt. Lloyd Baynes of Richmond said approvingly, as he looked over the 23rd Virginia Volunteers assembled at New Market, Va., recently. ("Farb" is a scornful term used for those who show up at the history recreations wearing polyester jeans with creases.) The 23rd and other Confederate units have their own newsletter - the Reb - and jobs that allow them to get away weekends and to sink sometimes as much as $500 in uniforms and equipment. They make their own non-farb uniforms, sometimes with the advice on patterns of the Smithsonian. Some are married, and wives often come along to the reenactments dressed in crinolines and lace gloves.

"Camp followers or nurses," explains a private.

They've got the fever up north, too, but there it's more likely to be Revolutionary reenactments, though the 1st Virginia Regiment here has 30 members and a newsletter telling them where the battle action will be every weekend. Editor of the newsletter is Tom Deakins, an FBI editor during the week. His wife, Carol, is a consultant to the National Parks Service on colonial costume, and she supplies the patterns for the 1st Virginia.

In the Revolutionary units, crafts are big. Some members of the 1st Virginia even make their own shoes. The rest get their boots from Frye, which makes proper reproductions.

"There's more interest in the Civil War than the Revolutionary," says Captain Bopp. "This is hard to believe but there's an American Civil War group in Australia.The Confederacy is more popular than the Union and I feel that way myself. The Confederacy had fewer men and they pulled off some unbelievable things. And in the Revolution, it's harder to trace your ancestors.

"World War II is getting popular now, too," said Bopp, referring to reenactments of Allied-Nazi battles. "I think that's because muskets are so expensive. They're all imported from Japan and Italy and they cost between $125 and $400. World War II battle fans probably started as weapon collectors because that stuff is still available and decided they might as well use them."

Of course, it's harder to recreate a world war on fields where it wasn't fought. Almost all of eastern Virginia could be fenced off and declared a historical shrine to the Civl War. Boys growing up there can't help but feel the root pull. "They see the historical markers everywhere," says Sims. "Kids who see fire stations want to grow up to be firemen. It's part of the same thing."

Lacking the actual battlefield, the Civil War buffs usually replay the battle in regional state parks or ballparks, but not in national parks. Demonstrations from both wars are allowed in national parks, but no mock battles. This is said to be for the public's safety - you can get a powder burn from a blank - but there are indications that it is also because national parks don't want to seem to be promoting war.

Opposing generals draw up battle plans. In May, the New Market Battlefield Park was host to 900 Union and Confederate soldiers reenacting the 1864 clash in which 247 Virginia Military Institute cadets helped stave off a Yankee victory. Confederates outnumbered Yanks, as usual, in the reenactment. Civil War replays have such a hard time filling the Yankee ranks that more than one young war buff has both a Union and a Confederate uniform hanging in his closet.

"Every mother's son wants to be a Confederate," said one Union Soldier for the day who was born and bred in Virginia. "The war's been over 115 years. I'm not prejudiced. I can die well."

And die they do, mostly when they get tired. Battle reenacting can be rather sterenuous. Getting over fences in full regalia, storming a summit in the summer is hot work. But until the battle moves on and the skirmish is over, they never take a drink from the canteen. No spectator is allowed to see a dead soldier revive.

Loyalty to the outfit is strong. One young man moved to Ohio from Virginia and adverstised in a "help for readers" syndicated newspaper column to find a southern unit where he could wear his Confederate uniform. He discovered one that originally had been formed in the Virginia town where he was born. Michigan and Wisconsin both have Civil War units, but by and large the fans are on the East Coast in where the battles were fought. Five thousand men are playing Revolutioanary solider up and down the eastern Seaboard.

War buffe kown the war songs and they march to the rat-a-tat-tat of drummer boys who look just like the pictures in the history books. They know the drills and the rebel yells, and their skirmishes spreading across two or three fields and up ridges to cannon positions look so real it's sometimes a shock to see spectators wearing jeans and T-shirts.

Battle audiences cover as broad a spectrum as the soldiers, ranging from manual laborers to history teachers. They're as enthusiastic as the soldiers, following it all as if the home team were playing.

"It gets to you," says Sims.

It does. On the field designated as a parking lot at New Market, one spectator's Buick was mired up to the hubcaps in the same kind of mud the VMI cadets must have had to contend with when they lost their shoes on the march toward the enemy. The spectator isn't worried about his car right now. He's setting up his folding chair by the barn and watching eagerly as the Union company comes whistling down the lane straight into an ambush.

"I love history," he says.

You don't have to fight to have a good time. CAPTION: Illustration, no caption; Picture 1, The Fifth New York Volunteer brigade in New Market, Va.; By Elizabeth C. Mooney.; Picture 2, A Union company presents arms for inspection; By Elizabeth C. Mooney.