FORT BENNING, GA. -- Pvt. Christopher Cashman was slinking through the woods stalking the sniper when he saw a puff of smoke from inside the woodpile dead ahead of him. Then Cashman heard the bang and the ringing of sensors on his body, confirming he had just been "killed" -- by a harmless laser beam from the sniper's rifle.

"I hit the ground," Cashman recalled. "I pounded the ground with my fist, cursed myself and asked 'Now who's going to take care of my family?' I was angry at myself for getting killed, angry for letting down the people behind me."

Cashman was leading fellow infantry trainees through the woods during a training exercise made realistic by equipping both the mock Soviets and the U.S. infantrymen with weapons that fired laser beams that set off sensors when they hit their mark.

"After I calmed down," Cashman said later, "I vowed that if it was ever the real thing, I wouldn't try to be a Rambo and expose myself."

Cashman drew the right lesson from the exercise, according to commanders here.

Army leaders say they want infantrymen who can shoot straight while lying flat, not Rambos who would get killed any time they stayed standing for more than five seconds while under enemy fire.

And it is clear that if today's "thin red line" of active duty troops is to win a small war or hold out against an enemy long enough to be reinforced by reserves and draftees in an all-out, nonnuclear conflict, the new GIs making up the post-Vietnam, all-volunteer Army must train in new ways to learn the hard lessons about how to survive against modern weapons.

After 13 weeks of training at this home of the infantry, Cashman did so well in learning how to survive that just before graduation here recently, he was one of four graduates in Delta Company, 2nd Battalion of the 54th Infantry to be promoted one rank.Typifying the New GIs

Cashman, 26, typifies thousands of new GIs in today's Army who tried several civilian jobs before deciding they could do better in the military. He and several others here said they chose the dangerous job of infantrymen as a way to get bonuses for signing up and quick promotions.

"I came home to California after playing rugby for 11 years all over Australia," Cashman said, recounting his path from civilian to Army life.

"I found my old friends had passed me by while I was away. They had finished college and had good jobs. I was married, had one child, and we were about to have another. I couldn't get a job anywhere in which I could make enough money to pay the rent in California or even buy a car. I decided joining the Army was the best thing for us."

The Army tried to reinforce the combat effectiveness of such motivated, volunteer soldiers by keeping them together in one outfit for at least three years.

The idea was to develop the kind of foxhole loyalty and teamwork that would enable U.S. forces to win against larger forces, like those the Warsaw Pact could send against NATO in Europe. This Army experiment at togetherness is called Cohort.

Cashman and 65 other infantrymen will continue their training at Fort Riley, Kan., forming the heart of a Cohort company slated to replace another company now in Europe with the 1st Infantry Division, the Big Red One.

Cashman's Cohort company will stay in Europe two years, in accordance with the ideas championed for the past eight years by the Army's former chiefs of staff, Gens. E.C. Meyer and John A. Wickham Jr.

But the nation's new Army chief, Gen. Carl E. Vuono, is stepping back from the experiment because managing it proved difficult, especially in Europe, where 217,639 U.S Army personnel are among the 325,565 American military assigned to NATO forces in Europe.

"We started Cohort," Vuono said in a recent interview, "because everybody said, 'When we were in Vietnam, and we had guys flown in and flown out, it was hard to build any cohesion, any buddy system.' Anything you do in terms of cohesion bonding helps.

"The old days of individual replacements in a war is the last way you want to do it."

But keeping European Army units up to strength under the Cohort replacement system has proved a nightmare, critics say, rather than the dream Meyer and Wickham had envisioned. Women cannot be used to fill the ranks because they are barred by law from combat units.

So instead of replacing one 125-man company with another in Europe, Vuono said, he will fill vacancies there thrice yearly, sending four or five men at a time in a "package replacement system" involving teams that have been working together in a rifle platoon or tank. The commander on the ground will decide whether to keep the replacements together or distribute them singly.A Good Riddance

Many officers and sergeants involved with the Cohort experiment said during interviews that they are glad to be rid of it.

They complain that Army personnel chiefs, for fear of shredding togetherness, did not fill vacancies in Cohort units caused by sickness or transfers. The result was that many Cohort units lost strength during the three years they were kept together. Also, the critics said, many young officers and sergeants felt they were stagnating in Cohort units and could not get the transfers to assignments which would enable them to compete with their peers for promotions.

Vuono said Cashman's will be the last company sent to Europe as an entire replacement outfit. While conceding that Cohort has proved impractical for Europe "at least in the near term," Vuono said he still intends to replace one company for another in South Korea, where tours of duty are only one year.

Instead of experimenting with innovations like Cohort to squeeze more combat power out of existing forces, contended a number of Army leaders involved with training today's soldiers, Pentagon generals should face the fact that the present active duty Army of 781,000 men and women is not large enough for the challenges at hand, especially if nuclear weapons are withdrawn from Europe under an intermediate-range nuclear forces treaty.

"Wickham was right in forming light divisions which could go to places like the Persian Gulf in a hurry," one officer said. "But he shouldn't have taken the men out of other units. He should have raised the end strength above 781,000. We're too thin, especially if we had to expand the training base to train a flood of draftees."

For World War II, the Army expanded from 218,000 active duty soldiers in 1940 to a peak of 6 million in 1945; for Vietnam, from 969,000 in 1965 to a peak of 1.6 million in 1968.The Two-Track Solution

Said Charles C. Moskos, Northwestern University sociology professor who has specialized in studying military manpower needs, "If we are truly concerned about strengthening our military deterrence capability while moving away from a reliance on nuclear weapons, this means coming up with an inexpensive, large supply of manpower."

Given the reluctance of the president or Congress to return to draft calls and the prospect that recruiting efforts could become too expensive to continue under a no-growth Pentagon budget, Moskos recommended that the Army embark on a two-track approach to volunteers. One track would offer two years' service at relatively low pay in exchange for college money. The other would sign up professional soldiers for tours of four years or more, at much higher pay than the two-year "citizen" soldiers.

"Sure, I'd come in for less pay as long as I could still get the college money," said Pvt Kevin Squires, 18, of Holmes Beach, Fla., who graduated with Delta Company at Fort Benning early in October. His views were typical of other two-year enlistees. Under his enlistment contract, Squires will receive $17,000 for college at the end of his tour. He will have contributed only $1,200 and the government the rest. The Army still offers two-year enlistments, depending on requirements.The Paradox of Deterrence

How big America's peacetime standing Army should be "is a real paradox," said Wickham, who retired as Army chief of staff in June. "It turns on how you define deterrence." That question would be pushed front and center by a treaty on intermediate-range nuclear missiles, he agreed.

"We're always going to be manpower-limited," Wickham continued. "Today's Army should be regarded as a seed-corn Army," one which could be used to grow a bigger force in times of emergency.

"You can't just look at the 781,000" on active duty, Vuono said. "You've got to look at the reserves" -- 424,000 National Guard "weekend warriors" and 285,000 Army reservists. "You're not going to fight over there {Europe} alone. You're going to fight as part of a coalition force." Today's Army "is big enough with mobilization. We're going to fight as a total Army both active and reserves. I see no time on the horizon when we would increase {the standing Army} any great amount."

Rather than grow a bigger Army, Vuono and other generals in Washington said, they intend to combine the new, smart, highly motivated GIs with the new lethal weapons the Army is buying to build more combat power into existing forces. With such a combination, the Cashman and Squires of the post-Vietnam Army would be able to fight the Warsaw Pact according to Wickham, as portrayed in the novel, "Red Storm Rising."

"I hope the Russians have read the book," Wickham said.