Jose Orozco, 59, is a wrinkled civilian, working as a mechanic in the Antilles Steel Mill just southeast of Havana.

But he knows enough about weapons to explain why a Soviet-made AK47 assault rifle is better than the Chinese-made model. The problem, he explained, is that the barrel of the Chinese gun heats up during full automatic bursts, while the Soviet weapon stays cool enough to hold even during prolonged fire.

Orozco, his friend Enemelio Trujillo, also 59, and about 80 percent of the factory's 4,600 other employes, men and women, have learned their gun lore from practical experience in Cuba's expanded Territorial Troop Militias, a nationwide rear guard designed to mobilize Cubans with weapons and training in every corner of the country.

President Fidel Castro's government, girding against what Cuban officials say is danger of a U.S. invasion, since 1980 has been involved in a concentrated effort to improve militia weaponry, training and manpower. In the past year, officials here say, it has topped the target of more than 1 million militia members with training and guns, representing more than 10 percent of the island's population.

"If a war comes, what do we do?" Castro asked in an interview last January. "All executives, all workers, all political, administrative and technical officials, everybody has the necessary training, the appropriate mission. It is impressive."

The Territorial Troop Militias have been organized in small squads to back up Cuba's 155,000-man regular armed forces and 190,000 reservists in time of war, officials said. Some have been assigned to protect neighborhoods, areas or factories. Others are on call to bolster regular or reserve units wherever the need arises, members explained.

Government officials said the main source of their concern is the United States, despite denials by the Reagan administration that it has plans to attack Cuba. The idea of the militia expansion, Cuban officials explained, is to raise the cost in blood of any such attack by spreading armed units around the country.

"It would be much easier for an enemy country to attack us if they knew we were unprepared," said Trujillo in a government-arranged interview.

The quality of weaponry has increased markedly in the last few years with the new emphasis on the militia, he and Orozco said. U.S. analysts have estimated that Soviet arms deliveries jumped to 66,000 tons in 1981, the first year of the militia buildup, after a decade-long annual average of 15,000 tons in the 1970s.

In addition, the government, expressing concern about a U.S. attack following the invasion of Grenada in October 1983, launched a campaign last year to encourage Cubans to build trenches and bomb shelters.

Although the campaign was proclaimed loudly then, there is little visible evidence of such shelters in Havana now. Several were seen in schoolyards and the gardens of public buildings, however, during a recent visit to the Isle of Youth off southwestern Cuba.

Most militia members already have completed their regular three-year military service. As a result, many have received military training before entering their units and have been required only to keep it up in the militia.

Orozco and Trujillo said they report for training one Sunday a month, leaving their houses in uniform to board buses at predetermined points and ride to military camp for "combat preparation." The day's training usually lasts until midafternoon, they said.

Weapons have been stored in camps or other deposits for use in emergency and are not taken home, they added.

The two steelworkers said they also take a full-time military training course of one month or more in rotating stints that usually come up about every 30 months. Women serve in separate units.

Most militia members have learned to fire weapons, particularly the Soviet-designed AK47 assault rifle that is standard issue in the Cuban armed forces, Cuban officials said. Some units also have received mortar and antiaircraft training, they added. Training comes from fellow militia members rather than regular Army officers, they said.

The expanded militia has never been tested in battle, although exercises have been held with the Army. An earlier version of the militia was on guard at government buildings and factories during the Bay of Pigs invasion in 1961.