The Pentagon, after a heated internal debate, has just decided to bet $1.66 billion that it can make a "smart" artillery projectile called Copperhead live up to its name despite all of the tests it has flunked.

The argument about Copperhead, a rocket-assisted artillery round designed to ride a laser beam to a moving enemy tank, illustrates uncertainties facing the United States as it decides where to invest limited money and talent to try leap-frogging the Soviet Union in high-technology weaponry.

Copperhead, despite its difficult birth, presents an easy case compared with President Reagan's recent request in his so-called "Stars Wars" speech for weapons to stop Soviet missiles in flight.

Copperhead "is the beginning of a new world of munitions," Undersecretary of the Army James R. Ambrose said Friday in explaining during a lengthy interview why he and others have rejected advice to abandon the controversial weapon.

"The camps are divided," he conceded, saying he and Gen. John A. Wickham Jr., whom Reagan has nominated to become Army chief of staff in July, "drove it back into the budget" after it was deleted to free money for less uncertain projects. "If we ran away from this one, Congress would give us hell."

Copperhead has yet to demonstrate in test flights that it would work on the battlefield, according to the privately financed pressure group, Project on Military Procurement, which released a thick package of Army documents showing how poorly the weapon has performed so far.

"There are inherent problems with this weapon, especially the laser, that probably cannot be fixed," the project said in assessing documents on Army field tests conducted in 1979. The group said more impressive test results achieved recently were conducted under controlled conditions that would not be matched on the battlefield.

Ambrose said he had heard the horror stories about Copperhead tests but had concluded after reviewing the weapon that "it is fundamentally sound." He blamed the weapon's contractor, Martin Marietta Corp., for failing to give "meticulous attention on the production line."

"I can't say contractor performance was illustrious," Ambrose said, but he added that major improvements have been made over the last three years. Ambrose said that when he learned the Copperhead contract allowed the weapon to be successful only half the time, "I said that's ridiculous."

"No matter what the contract says, you can do better and you ought to do better. This thing ought to hit 80 percent to 90 percent" of the targets, Ambrose said he told the contractor.

Martin Marietta agreed, Ambrose said, and kept working on quality control and replacing unreliable parts until recent tests achieved reliability of 90 percent or better. The contractor has also promised to deliver 30,462 rounds of the Copperhead for no more than $1.658 billion, or about $54,428 for each round shot out of a 155 mm howitzer, Ambrose said. "It's a hell of a price," he conceded, "but the thing you're trying to kill costs $1 million or $2 million."

The Army's brochure on Copperhead said "its accuracy is so good that it can literally drop down the open hatch of a moving tank" when fired from 10 miles away.

However, Copperhead's backers and critics agreed that the job of making the wonder-weapon accurate falls to a lonely soldier on the forward edge of the battlefield. He must crouch in an exposed position to focus the laser beam briefly on the tank so beams will be reflected skyward where the Copperhead will sense them in flight.

The soldier must also carry on a conversation with the gun crew firing the Copperhead from a rear position.

The Project on Military Procurement complained that the Copperhead has a minimum range of one kilometer and cannot be used if a tank is closer. "Thus," the project said, "it will be unusable for approximately 85 percent of real tank engagements."

Official Army reports on Copperhead tests collected and released by the project made these other criticisms:

* The laser designator that the soldier must aim at the target "is too bulky and heavy" and "presents too high a silhouette and requires the operator to assume a sitting or kneeling position."

* The laser-Copperhead combination "does not function in anything but perfect weather" and would have limited use in overcast conditions often found in Europe.

* Coordination required between the soldier up front and the artillery battery in the rear is "excessive," especially for "a fast-moving battlefield or in high-stress situations."

The Army originally intended to buy 180,000 Copperhead rounds but has reduced the order to one-sixth of that, driving up the cost of each projectile. Despite the extra cost, Defense Secretary Caspar W. Weinberger told Congress last week that he considers it vital to the national defense to go ahead with Copperhead.

"There are divided camps," Ambrose said of Copperhead. "It's a judgment call by management" to go ahead. "It's way past time for the Army to get into the smart munitions business; we need to learn the field. We don't have a loser in the sense of having a dog that won't work."