Five experts in chemical warfare said yesterday that there was no evidence the Soviets are ahead of the United States in making or stockpiling chemical weapons and the Reagan administration's push to build new nerve-gas weapons could threaten plans to build up conventional forces in Europe.

The experts, all opponents of the new chemical-weapons program, opened a two-day hearing before the Senate Appropriations Committee,attacking each of the points the military has used to argue for the new "binary" chemical weapons.

The Reagan administration is seeking $54 million in fiscal 1983 to begin production of binary artillery shells, so called because they contain two separate components that mix after the shell is fired to form lethal nerve gas. Pentagon officials argue that for this reason, the binaries are safer to handle and store.

Matthew Meselson, a biochemist from Harvard and chemical weapons consultant to each administration since John Kennedy's, said the United States had enough live nerve-gas shells to wage war in Europe with regular chemical barrages for at least 90 days and to supply all NATO forces as well.

But using that much chemical weaponry is unlikely because chemical weapons have "the two most terrible" traits any weapon system can have, Meselson said.

"They will cause immense civilian casualties . . . and I do not believe they would be at all effective" because the Soviets would simply don their protective gear and fight on, he said.

Several witnesses pointed out that chemical weapons are no longer considered useful to "produce casualties" in war, but are merely useful to force one's opponents to suit up in protective gear.

Julian Perry Robinson of the University of Sussex in England testified that another consideration is that Europeans are already very sensitive on the subject of storing or using American chemical weapons on their soil.

A new round of producing chemical weapons would "inflame . . . European sensibilities even further," he said.

It also would threaten readiness for conventional war, he continued, because it would "undermine and even in some cases destroy the delicate political compromises which have been reached on upgrading conventional weapons" in Europe.

Sen. Jake Garn (R-Utah), who favors the new chemical weapons, disputed the argument that they are needed to replace old ones that have deteriorated and are leaking. He said that there had been no accident in 35 years of stockpiling the weapons in his state.

A few internal parts of nerve gas weapons were found to be leaking, he said, but the amounts were so small "that if you put a human being inside a canister outer casing and sealed him up for eight hours with the leak, there would be virtually no damage." He said the leaks amounted to just a few parts per billion.

Meselson said that "every single lot" of the nerve gas shells that would be superseded by the new weapons is listed as in top condition and ready for immediate shipment and use according to the Army's own figures.

To make new shells of this kind would cost $550 per shell, compared with only $20 to $30 to maintain each of the current ones.

As for the safety issue, each witness said there had been no significant safety problem in 35 years of nerve-gas storage. Meselson added that the new binaries would be bulkier to ship and more difficult to assemble than the current "rugged, simple" shells.

Appropriations Committee chairman Mark O. Hatfield (R-Ore.) also contended that at a time of budget austerity, "producing chemical munitions may mean forgoing other steps aimed at enhancing our national security, including other conventional defense priorities."

Also testifying were James F. Leonard, a former senior official in the Arms Control and Disarmament Agency; retired Rear Adm. Thomas D. Davies, former assistant director of ACDA, and Saul Hormats, who directed development of the Army's current chemical munitions and protective equipment.