The Soviet Union has begun flight tests of a successor to the SS20 nuclear missile, indicating that further deployments of the 18-year-old weapon were doubtful even before Soviet leader Mikhail Gorbachev's announcement Sunday of a moratorium on intermediate-range missile systems, according to experts inside and outside the U.S. government.

This information suggests that Gorbachev's plan to halt further deployment of such systems until November comes at a time when the Soviets may be switching from an older missile to a more modern one.

The new Soviet missile has been given the designation SS-X-28 by the Pentagon, according to one source. A brief reference in "Soviet Military Power," the Defense Department's annual review of Soviet weaponry, refers to a modified SS20 that will "have even greater accuracy and other improvements over the current SS20."

If the Gorbachev moratorium referred only to further SS20 deployments, it was "like him saying the Soviets would stop doing what they were planning to stop doing anyway," Steven Meyer, an expert on Soviet weaponry, said yesterday. Meyer, a consultant to U.S. government agencies, is an associate professor at the Massachusetts Institute of Technology.

He said the rhythm of Soviet missile production over the past 25 years indicates that a modernized intermediate-range missile with better accuracy and reliability than the SS20 is long overdue.

The SS20, designed in the mid-1960s, first appeared in 1976 or 1977 aimed at China and deployments have continued in the succeeding years, Meyer said.

According to Reagan administration officials, the Soviets have deployed 414 of the three-warhead, road-mobile missiles at bases across the country. With their 2,500-mile range, the roughly 270 SS20s west of the Urals could hit targets in Western Europe and North Africa. The remaining SS20s in central and eastern Soviet territory could reach China, Southeast Asia, Japan and most of Alaska.

The deployed missiles, which carry more than 1,200 warheads, have been "far more than you could rationalize for military purposes," Meyer said yesterday. He suggested, and government sources agreed, that the last 50 to 100 SS20s deployed over the past two years were "for political purposes, to match American deployments" of Pershing II and ground-launched cruise missiles.

In 1979, the NATO alliance approved a plan to deploy 108 American Pershing II and 464 cruise missiles, beginning in 1983, when the first missiles would be ready.

The U.S. missiles were designed to renew and strengthen America's nuclear commitment to the defense of Europe and to balance the introduction of the SS20s, which in 1979 numbered 75. NATO experts predicted that the Soviets would eventually deploy about 300 SS20s with a total of 900 warheads.

The number of U.S. missiles was set at 572, not enough to present a serious first-strike threat to the Soviets. The range of the Pershings was also limited to 1,000 miles so they could not reach Moscow.

During the four years between the deployment decision and arrival of the first American missiles in Europe, the United States and the Soviet Union were to try to negotiate limits on the new Euromissiles.

From the first discussion by the allies of the new American systems, Moscow bitterly opposed the plan. Before NATO's approval of the "two-track decision" calling for both negotiation and deployment, then-President Leonid Brezhnev offered to reduce Soviet missiles unilaterally if no U.S. Pershing or cruise missiles were sent to Europe.The NATO allies rejected that approach.

In 1980, during abbreviated negotiations on intermediate-range weapons under the Carter administration, the Soviets offered to freeze SS20 levels in Europe while remaining free to deploy additional missiles in central and eastern areas.

The Reagan administration in 1981 made its zero option offer, proposing in the negotiations that the United States would drop its plan to put missiles in Western Europe if the Soviet Union destroyed all its SS20s.

Brezhnev's response at that time was to call for a moratorium by both sides on deployments of all such weapons while talks were under way. Faced with U.S. rejection of that approach, a year later he offered a unilateral freeze of SS20s, if U.S. deployments were delayed. It was an offer similar to the one made Sunday by Gorbachev.

In 1983, with the first American missiles scheduled to become operational in December, American negotiators offered to settle on any equal number of U.S. and Soviet missiles, between the president's zero and the 572 of total deployment. While Moscow agreed to reduce its SS20 force to 162 from the 300 then deployed, it held to its demand that no U.S. missiles be put in place.

In November, when the West German parliament approved the American Pershing deployments, the Soviets broke off the intermediate-range weapons talks. A month later, when the first cruise missiles arrived in England, the strategic arms talks were also ended.

The first American deployments consisted of 16 cruise missiles at bases in Italy and England, and nine Pershing missiles in West Germany.

Since that time, the Pentagon and NATO policies have been not to announce deployments.

Currently, according to sources inside and outside government, 54 to 63 Pershings are deployed with the remainder of the planned 108 scheduled to be operational by December of this year.