In a scrawny Virginia forest this week, the men of B Battery. 108th Field Artillery, 28th Infantry Division, U.S. Army, are being taught the latest techniques for using small connon in conventional warfare. Meanwhile, back in their armory in Gettysburg, Pa., two perfectly functional $24,000 cannon sit, unused and unattended.

On the sandy soil of Ft, A. P. Hill, four of B Battery's 105 mm howitzers face a row of exaciated pine tree. When the order to fire crackles over the radio, three of the howitzers emit ear-ringing blasts, sending their 33-pound shells speeding toward a target five miles away.

The fourth howitzer will not be fired, the battery commander explains, for the same reason that the other two were left behind in Gettysburg: the battery does not have enough men to use them.

"Last year we could man all sex of them," said Capt. Ronald Baker. "This year, we con man three . . . Our goal is to become combat-ready, but, due to some manpowrr problems, we'll never do it."

Baker and his men are not soldiers.

They are bankers, teachers, construction workers, policemen, doctors and others who spend one weekend per month and two weeks each summer training as members of the National Guard.

There are supposed to be 89 men in Baker's company. There are 66. There are supposed to be eight men operating each gun. Baker operates with four or five per gun.

"What that does, it wlows us down somwhat," said Baker. With the proper number of man, B Battery could fire two or three rounds per minute, and benefit from the practice that is the heart of their training; as it is, they con fire only once every two or three minutes.

"That's how it is," Baker said, "in the National Guard."

The National Guard. A place for men too old for regular military service, playing out their active years as part-time soldiers; for kids lucky enough to evade the Vietnam-era draft by squeezing into the Guard rather than going to Canada, or to prison?

Maybe. There are examples enough of ineptitude in B Battery, one of the better guard companies, according to its battalion commander. There is the case of the four men who were sent to camp early, only to find they had nothing to do but sleep - and get paid - for three days.

There is the sergeant who could not get his howitzer leveled because he kept lowering the side that was already too low, and the private who picked up and unexploded shell and thrust it at his lieutenant, saying, "Look what I found."

But the men of B Battery work hard, and this country's whole national defense strategy is based on keeping them, and other guard and reserve units, ready to go to war in a hurry.

If the U.S. Army becomes engaged in a major conflict tomorrow, 58 per cent of its field artillery, the major killer of enemy personnel, will come from the National Guard or the Army Reserve.

So will 65 per cent of the Army's combat engineer battalions, 52 per cent of its infantry and armor battalions, 45 per cent of its aviation forces, and 65 per cent of all the Army's tactical support.

"There was a time when you joined a guard or reserve unit and kind of sat around, didn't do much as far as effort was concerned," said Will Hill Tankersley, outgoing deputy assistant secretary of Defense for reserve affairs.

"It's not a funny business anymore," said Tankersley. "It's not a joke. The survival of our country depends on them, and the margin for error's gone."

The National Guard and Army Reserve find themselves today at a curious crossroads. In the planning rooms of the Pentagon, where the national military posture is worked out, the role of the reserve components has been assuming an increasingly high profile over the past five years as the size of the standing force, has dropped.

Of the Army's 24 divisions, eight are either reserves or guardsmen. In a war situation, more than half - 54 per cent - of the men the army could put in the field would be from the reserve component. The reserves' role even includes the crucial refueling of some of the long-range strategic aircraft in the Air Force arsenal.

At the some time, the guard and reserve are suffering a devastating and worsening manpower shortage. As of April 30, the guard with a congressionally authorized strength of 390,000, had only 357,570, or 91.7 per cent f the number Congress thinks it needs to perform its tadk adequately.

The numbers are in a precipitous decline. During the month of April this year, the guard lost 3,127 men, while the reserve lost 591. The Marine Corps Reserve, also under 90 per cent of its authorization, lost 61 men in April.

Even these figures are somewhat deceptive. During the transition quarter, between July and October of last year, Congress, in a first recognition of the reserve manpower problem, reduced the authorized strength of the guard from 400,000 to 390,000 men, and of the Army Reserve from 219,000 to 212,400.

There are more substantial solutions to the manpower problem floating around Washington.

Surprisingly, in the absence of a draft, the reserve components have little trouble getting recruits in the front door. The problem is keeping them in once they have joined.

The solutins fall into two basic areas. One is to improve the training, in order to bring the reserves to combat-readiness, and, equally important, to make training more fun.

The other is to re-order the financial incentives offered potential re-enlistees, so taht they would receive additional benefits at the time of re-enlistment, rather than at the end of their tours, lwhen traditional GI Bill benfits come.

Asked if the reserve component is now deficient in its ability to perform its assigned national defense role. Tankersley replied. "I think so. It's like asking a man to bake a cake, and giving him the money to buy only half giving him the money to buy only half the sugar he needs, or half the flour . . ."

It was not always this way. Only half a decade ago, manpower problems were the last things the guard and reserve contemplated. Impelled by the draft, and the prospect of Vietnam, young men besieged the guard and reserve for the few precious spots that could mean a safe and respectable way of avoiding a future filled with monsoons, black pyjamas and land mines.

The draft ended at teh beginning of 1973, and the last group of people who joined the guard or reserve under Vietnam pressure are just now coming to the end of their six-year obligations.

Most of them are getting out as soon as they are able. The manpower decline. Tankersley told a House Armed Services subcommittee in March, "has been caused primarily by the large numbers of draft-motivated individuals who enlisted in the late 1960s and early 1970s and who have elected not to re-enlist as they completed their six-year military obligation."

Recruiting remains possible in today's atmosphere, when the military is soon as less of a parish than it was during the height of Vietnam, but the new recruits, like the draft-era enlistees, do not stay.

Last year, for example, the National Guard enlisted 108,000 new recruits, the second highest number sonce the end of World war II. But it lost 135,260 men during the same year. And the energy that must be divoted to recruiting is burdensome.

"While we were getting those 108,000, the commanders and their team leaders were spending and inordinate amount of their time recruiting that they rightfully should hav espent training and working with their units," said Maf. Gen. La Vern Weber, top man in the guard.

"Strength is critical to us because we're spending too much time recruiting, and not enough time training and administering. That's really our problem," Weber said.

From the upper reaches of the Pentagon to the men who pull the lan-yards on B Batterys 105s, there is near-unanimous agreement that improved training is the dey to deeping men in the guard and reserve.

"People don't quit the guard and reserve because it's tough," said Tankersley. "They quit because it's boring." One sllution, he said, is "interesting meaningful, challenging training."

By most accounts, training in the reserve components had improved in recent years. With their new roles in the total defense posture, the guard and reserve now have a mission that, officials say, is communicated to their men through realistic, hads-on training that emphasizes war games and the use of weapons.

But the training is still not what it could be. It would not be difficult to set your imagination free in the woods of Ft. A. P. Hill, to turn the 6-inch-high grass and scraggly pine trees into a forest in Germany, or Korea, or Vietnam, to make of the aging, rusted trucks and helicopters that serve as artillery targets, bridges or roads that must be destroyed to portect your troops.

But training is not done that way in the National Guard. It "isn't very relevant,"said Staff Sgt. Thomas Steele, and artillery expert from the standing Army who is advising B Battery during its summer exercise.

"It's not factual training (involving a situation) like you'd hav ein combat," Steele said. "They don't say, "You're supporting an attack, or a retreat." They say, "Aim at point 44 and shoot . . . '"

The guard training is disturbing to Steele, a Vietnam verteran, and if Tankersley is right, it should be. "Teh regulars realize that if they have to go on the battlefield in Europe," he said, "their lives will depend on a guard or reserve battalion."

There are several proposed solutions to the reserve manpower problem. Pending in Congress are bills that would provide educational benefits and increased financial incentives to re-enlist after the completion of the six-year obligation.

More money has been allocated for recruiting this year than ever before, and the problem is being discussed throughout the military establishment.

"A willtrained, porperly equipped, highly motivated guard and reserve of sufficient size and quality is not one of several alternatives." Tankersley told Congress in February. "It is the only alternative to have an adequate total force to meet our defense needs."