Psychiatric experts who examined John W. Hinckley Jr. told Congress yesterday that proposals to change the insanity defense law are unwise.

Some bills to change the insanity law before Congress would shift the burden onto the defense to prove the accused was insane; others would eliminate the adversary system of psychiatric testimony, and many would establish a guilty-but-insane verdict.

Dr. Jonas Rappeport, chief medical officer for the Supreme Bench of Maryland who examined President Reagan's assailant for the prosecution, said he saw no reason to change the law.

"I believe the insanity plea as it currently exists in most states and federal jurisdictions requires no alteration," he contended. The law, which allows the jury to find someone not guilty if insanity prevented him from knowing he was committing a crime or from controlling his actions, "is the mark that separates us from wild beasts."

Dr. Ernst Prelinger, a Yale University professor of psychology who testified for the Hinckley defense, told a Senate Judiciary subcommittee that proposals to have only court-appointed experts testify was a poor idea. He said having both sides supply their own experts is "best done in the adversary system."

Also testifying was Dr. James Cavanaugh of Chicago's Rush-Presbyterian Medical Center. He and Rappeport were government experts; Prelinger was hired by Hinckley's lawyers.

All agreed a bill before the Senate with 52 consponsors to create a verdict of "guilty but insane" is not practical.

The bill, scheduled for a hearing next month in the House where it has strong support, would send a person to a mental hospital if convicted and then to jail for a specified term once doctors certify he is sane.

"It makes no sense because of the practical effects," said Prelinger. "It would make treatment impossible because there would be no incentive to recover."

Prelinger said that shifting the burden of proof on sanity from the prosecutor to the defense would be unfair because it would create the only situation in the American criminal system where the burden of proof is on the defense.