President-elect Ronald Reagan's inaugural committee has abruptly scrapped its plans to have military marchers carry American flags rather than guns in the Jan. 20 parade, and is now considering plans to include 18 times as many military persons as originally planned.

The decision is a direct turn-about from plans committee cochairman Robert K. Gray disclosed last week that are said to have angered military officials and some top Reagan advisers, including Gray's cochairman, Charles Z. Wick.

In order to smooth some military feathers, the committee now has decided to allow military personnel in the parade to march with sidearms and rifles. In addition, parade chairman Terry Chambers is considering inviting 300 persons from each of the nation's five military academies, instead of the 81 from each school, as originally planned.

Last week, Gray told a reporter, "It would seem out of place to have tanks or missile-launchers in the parade and the governor [Reagan] believes the same is true about sidearms. We are a patriotic nation, not a militaristic one."

Yesterday, however, Gray said he had never discussed the no-guns policy with Reagan or Wick.

"I assumed that he [Reagan] would realize that they couldn't carry both [guns and flags]," Gray explained yesterday."

Gray said that after he announced that flags rather than guns would be carried, military officials and parade chairman Chambers told him it was "not practical" to have close marching troops all carrying flags because a breeze "could cause the flags to wrap around the people's heads or could snap someone in the face or even put out an eye."

Because of the danger, Gray said, he decided to drop the idea.

Inaugural sources said military officials strongly objected to Gray's flag suggestion. Gray was told by one official, a source said, that "marching in the parade without our guns is like walking the street with your fly open."

Wick also was not happy with Gray's suggestion. "I think it would be wrong to deviate from traditions without some strong symbolic reason and this is not such a reason," Wick said yesterday.

"Governor Reagan has stressed military strength, preparedness, and morale -- peace through strength. This [change] might suggest an amasculation of that strength," Wick said.

"This has all been very unfortunate," said Gray. "It was a good idea, it would have been very visual and graphic to have an Avenue of Flags coming down the street, but the whole thing is getting blown out of proportion."

Gray also announced yesterday that the committee expects to have 50 black stallions near the front of the parade with riders carrying flags from each state.

"We're still going to have plenty of flags," he said. "We might have to put some along the street, too."