At 12:43 a.m. yesterday, a bullet ripped through the body of the midnight clerk at an Arlington 7-Eleven store.

The bullet then punched a thumbsize hole in a Plexiglas window near the Big Gulp cups and the Playboy magazines. The two robbers, who didn't get any money, got away.

It was just another convenience-store robbery, one of 40 this year in Arlington and one of scores that occur year after year in the Washington area.

But Joseph Blas, 26, an Army staff sergeant moonlighting to make ends meet for his wife and two children, had never been robbed or shot before. His wife Vella, a thin 21-year-old woman new to the Washington area, had never been awakened by a phone call in the middle of the night telling her to get dressed for a car ride to the hospital.

Vella Blas had never felt the numbing fear of meeting a surgeon in a brightly lit hospital corridor, a surgeon who was rushing to operate on her husband and who only had time to say: "I think I can save his life."

Joseph Blas, a member of the Army's Old Guard at Ford Myer and a part-time clerk at the 7-eleven store at 5122 Wilson Blvd., was listed in serious condition last night at Arlington Hospital. He is expected to recover.

But his abdominal wounds are extensive, his future in the Old Guard as a leader in cermonial funerals is in doubt and his wife has no idea what she and her children, ages 1 and 2, will do for Christmas.

"We feel hurt, you know. When you have something like this. Doesn't look like there's going to be a Christmas now. We'll just sit around, I guess," said Mrs. Blas.

According to Arlington detectives, two young men walked into the store, approached the counter and chatted with blas about the prices in 7-Eleven stores. Then one of them pulled out a pistol -- believed to have been either .32- or .38-caliber -- and ordered the clerk to turn over all the money.

Police say the two may have been the same two who hit five other convenience stores in Northern Virginia in the last three weeks. Until yesterday, no one had been shot.

At 7-Eleven stores, according to a regional supervisor in charge of the 450 stores in the Washington region, clerks are taught not to give robbers a hard time. "I hate say it, but we run all our robberies like sales transactions," says Scott Valentine, the regional supervisor. "We train our people to turn over the money as fast as possible."

The store on Wilson Boulevard, however, didn't have much money to turn over. There was $18 in the cash register. The rest of the store's money was in a newly installed, computer-activated safe, the Tacc-I Timed Access Cash Controller. The safe is designed to give out a maximum of only $10 every two minutes.

Police said that for some reason-- perhaps because the robbers became impatient with the stingy electronic bank -- the robber with the pistol shot Blas at point-blank range through the stomach. The robbers then ran outside where witnesses saw them jump into a car and speed away.

Arlington police Det. E. W. Gabrielson tried to explain the shooting, the first in an area 7-Eleven store this year: "For some reason, either they (the robbers) didn't get what they wanted or the the victim tried to buck."

Blas, the only person left in the store, managed to reach a telephone beside the cash register and call police. He gave only sketchy details of the shooting before an ambulance took him away.

Blas was filling in yesterday morning for the regular midnight man, Willie Wright, who was home with pneumonia. Blas was making $3.20 an hour and had been working nights for two weeks while working his Army shift.

"When he got off his 7-Eleven job," said his wife yesterday, "he'd go straight to the Army. Then he'd come home from work about 5:30, sleep to about 10:30 and go back to the 7-Eleven. Nobody could tell him not to work."

Blas, who earns $1,115.70 a month as a grade E-6 staff sergeant, is one of hundreds of military people in the Washington area who moonlight to support their families, according to local military authorities.

"There is no housiing allowance, no special cost-of-living escalator here in high-priced Washington," said one of Blas' superiors yesterday. "They have to work at a second job to make ends meet."

Blas, who joined the Old Guard at Fort Myer this summer, has been working part-time for 7-Eleven since early October.

His wife ws awakened by the phone at 1:30 a.m. yesterday. A man's voice said: "Mrs. Blas, your husband has been held up. He was shot. We are sending a patrol car to take you to Arlington Hospital."

Before she could get her cloths on, a polcieman knocked at the door of her two-bedroom, $270-a-month apartment in Alexandria. She left her children, Carmen, 2, and Timothy, 1, with Blas' 16-year-old brother, who is visiting them.

In the squad car, the policeman couldn't answer Mrs. Blas' questions. The ride was almost without converstion.

At the hospital, Mrs. Blas was met by Scott Valentine, the 7-Eleven suprevisor. Together they went inside to hear Dr. Peter Lenard, the surgeon, tell them that Blas may survive.

"I went to a waiting room while they operated. I just sat there," Mrs. Blas said.

Vella Blas met her husband nearly three years ago when he was stationed near Jacksonville, N.C. She was then 18 and was standing in her grandmother's gravel driveway when he drove by and yelled hello. They've lived since in Louisiana and she came north to Washington for the first time in September.

"I'm working part-time at Woolco, We got enough money, really. He didn't have to work all those hours at 7-Eleven," she said yesterday. "But he never seemed to show being tired. Neither of us ever talked about him getting held up."

At 7:30 a.m. yesterday, Mrs. Blas was allowed to see her husband. He was unconscious and had a tube in his mouth. His wife looked at him briefly and went home, returning to the hospital last night.

The Southland Corporation, which owns the 7-Eleven stores, announced last night that it is offering a $10,000 reward for information leading to the arrest and indictment of the men who shot Blas. Arlington county also is offering an undisclosed reward.