Counterrevolutionary guerrilla commander "Krill" took cover and listened to the sounds of a firefight rising through the thick forest, studying them like a conductor searching out the notes of each instrument in an orchestra.

The firing of the FAL automatic rifles is so loud that in close combat the clatter of the Sandinistas' lighter weapons cannot be heard above them. "Ours," Krill said. The throaty burst of the M60 machine guns; the crash of the Light Antitank Weapon, a single rush of explosion--"Ours."

Then in a moment of silence the distant chunk of a mortar firing, the whistle, then the thunder of impact. "Theirs," he said, probably a Soviet 82mm.

Through much of the fight Krill was on the field radio talking to his forward units, verifying their positions, checking their movements.

Central America, long the scene of ragged rebel bands with a hodgepodge of equipment, has not seen or heard a guerrilla war like this before.

The tactics and techniques used by the U.S.-backed army fighting to overthrow Nicaragua's leftist Sandinista government are a blend of conventional and insurgent warfare that depend for much of their effectiveness on a level of firepower that the leftist guerrillas of El Salvador, for instance, generally do not approach.

Except for a handful of newcomers and women who carry captured Soviet-designed AK47s, rebel troops appear uniformly armed with the heavy, rugged and long-range FALs. They are Belgian-made and were standard issue in the Honduran Army before the recent increase in U.S. arms to that country.

The 80-man patrol with which I traveled for six days also carried two .30-caliber machine guns, one 60mm mortar, two M79 grenade launchers, a captured Soviet RPG7 rocket grenade launcher and, most impressively, eight of the U.S. bazookas known as LAWs, which are discarded after a single firing. One of these easy-to-use devices, aimed on a direct line and effective up to several hundred yards, can blow apart the East German trucks used to transport Sandinista troops.

From backpacks to boots to the anodized black knives that replace the traditional Central American machetes, much of the rebels' equipment is made in U.S.A., its olive green stamped by military outfitters such as Gibraltar Industries and Eastern Canvas Products.

The workmanlike forest green uniforms worn by most of the troops do not have any labels, but several soldiers said they thought they were supplied by Argentina.

The rebel soldiers said some of their materiel is captured from the Sandinistas, but most has to be carried into Nicaragua on their shoulders from neighboring Honduras. Many are accustomed to marches lasting days or weeks with plastic grain sacks full of cartridges on their backs.

One patrol commander said that supplies are occasionally picked up deep inside Nicaragua, where they are apparently dropped by helicopter or parachute. But in the area of Nueva Segovia, the rebels said, bringing in any sort of airlifted supplies was largely prevented by the terrain and, often, by the intensity of the fighting.

One Honduran officer who has worked with the rebels sees this logistical bottleneck as the greatest obstacle for the moment to the growth of the movement as it now exists.

Speaking along the same lines, Nueva Segovia's rebel commander Pedro Pablo Ortiz Centeno, known as "Suicide," said confidently, "We have the bodies, but we need the weapons."

The planned taking of Jalapa or a similar town with airstrips and highway access to Honduran supply points would solve that problem quickly. While this is their medium-term objective, they say their immediate aim is to inflict as many enemy casualties as possible and hold an ever wider swatch of territory.

The political leadership of the Nicaraguan Democratic Force, to which the troops here belong, now talks openly of an alliance with anti-Sandinistas in Costa Rica under Fernando Chamorro, who, they hope, will soon open a second front in the south.

The troops seen here were able to employ remarkably effective tactics based on the volume of fire they can lay down and the sophistication of their communications.

Each 80-man patrol is divided into four 20-man squads, which can work independently or together as the situation demands. In some cases these squads communicate directly on American FM/AN-77 field radios or similar equipment. There is also radio contact among the major patrols operating under Ortiz Centeno's leadership.

"Without communications I am nothing," he said.

As we traveled with Krill's units, at least once a day other patrols were informed of our whereabouts through coded coordinates copied and deciphered by the radio operator into one of the little school notebooks supplied by the Sandinistas for their 1980 literacy campaign.

The guerrillas of the counterrevolution move through the countryside in broad daylight, they said, because they can spot ambushes more easily that way. If one of the forward squads makes contact or the middle is hit or the rear is fired on, the others are close enough to enter the fight but far enough away to do it on their own terms.

When a small Sandinista detachment that had managed to follow us one morning undetected to a river crossing in a steep ravine began to take up position for an ambush, they were surprised by one of the patrol's advance squads firing on them from the opposite mountainside.

The rebels claim to have damaged or brought down several Sandinista aircraft with their M60 machine guns, for instance. While this could not be confirmed independently, not one Sandinista plane, not even an observation craft, flew within view during our visit.

Until recently, the rebels said, most of their fighting was done against militia units that were vastly inferior in both training and armament to their own ranks.

"Meat for the cannons," Ortiz Centeno said without emotion. Now there are more regular units of the Sandinista Popular Army being sent against them.

As a Sandinista mortar appeared to be bracketing his command position a few days ago and radio reports from his patrols brought him word of heavy combat on a nearby road, Ortiz Centeno smiled nonchalantly: "The ones who want to fight, you have to give them a fight."