Amid shouts of "Fire!" and the thunder of cannon, thousands of men in blue and gray stormed each other to re-enact the opening of the Battle of Gettysburg, 125 years after the Civil War's bloodiest confrontation.

A total of about 8,000 men representing Union and Confederate troops traded mock rifle volleys and cannon bursts Friday, ripping open paper gunpowder cartridges with their teeth while enemy lines closed in.The Confederate troops pushed back the Union's black-hatted "Iron Brigade," as scores of men on both sides feigned death in the tall grass and spectators watched from a hill overlooking the field.

"When it gets to firing, you're not in 1988 anymore, you're in 1863," said Jeff Douthit, a 28-year-old farmer from Unionville, Va.

The participants, among them teachers, engineers, artists and soldiers, had invested $1,000 to outfit themselves with the uniform, gear and guns needed to become Civil War soldiers, if only for a day or two.

Soldiers in the 23rd New York Regiment, played by a group from Arizona, went so far as to follow Union recipes down to "hardtack," hard, flat squares of bread.

"You might think it was made 125 years ago. It's that indigestable," said Ed Gouvier, a 37-year-old engineer from Phoenix.

Mentioning anything modern by members of the 23rd New York resulted in punishment: standing on a wooden crate at the end of one of the rows of white, canvas tents.

Gouvier said people joined the outfit for a chance to relive history, for the camaraderie, or to take part in the battles.

"I've always been a history fanatic," he said.

Others had more personal reasons for joining one of the ersatz armies.

Steve DiCarlo, of Owings, Md., said he became a member of the 18th Virginia Regiment in part because his great-great-great-grandfather was a captain in the Confederate Army.

"My wife thinks I'm crazy," he said.

Napoleonic Tactics Inc., which sold tickets to cover its $450,000 budget for staging three days of ersatz warfare, has billed the event as the largest Civil War re-enactment ever. The consulting company was hired by the event's sponor, the American Civil War Commemorative Committee Inc.

A total of 75,000 spectators were expected to attend the re-enactment, which ends Sunday.