The Carter administration should reappraise the national decision to rely on volunteers rather than the draft to fill the ranks of the armed forces, the chairman of the Senate Armed Services Manpower and Personnel Subcommittee said yesterday.

Sen. Sam. Nunn (D-Ga.) made that recommendation while discussing a report calling for a buildup of NATO conventional forces to combat the growing Soviet threat.

Nunn said some kind of national service for young Americans, included military service but not limited to that, "is generally the direction" he favors, especially since unemployment among young people is so high today.

Nunn thus became the second high-ranking member of the Armed Services Committee in recent weeks to express second thoughts about relying on volunteers to keep U. S. armed forces up to their current active-duty strength of 2.1 million. Committee Chairman John C. Stennis (D-Miss.) expressed his reservations at a Dec. 23 press conference.

Said Nunn yesterday: "I want to put our bureaucracy back to work to see what our options are" for switching away from the "all-volunteer-force" concept. To date, Nunn complained, the Pentagon has failed to define "what success is" as far as recruiting volunteers. The draft authority expired on July 1, 1973.

In a related development the Pentagon has renewed its effort of last year to get Congress to legislate standby draft machinery, although the latest proposal would be for use during war rather than as an alternative to the present system of recruiting volunteers in peacetime.

The United States needs a new standby " wartime draft system." which could produce 650,000 trained men within six months to replace those wounded or killed in a European war, according to a Pentogan memo to the White House.

The memo was sent in last days of the Ford administration to the White House National Security Council for President Carter's consideration, according to the Associated Press. "Performance rates in previous wars are too slow," said the meno in calling for a streamlined draft sustem for wartime.

Under the plan, 20-year-olds would participate in an annual lottery to determine in what order they would be called if war came. The system would be run by reservists attached to 66 regional offices rather than the draft boards comprised of local citizens.

The Pentagon's concern about gearing up quickly for a conventional war in Europe was shared by Nunn and Sen. Dewey F. Bartlett (R-Okla.) as they reported on their NATO survey taken between Oct. 31 and Nov. 14 1976, on behalf of the Armed Services Committee. They sounded these warnings in their report, entitled "NATO and the New Soviet Threat":

The Warsaw Pact has "deliberately retailored" its forces along the European front "to exploit fully the very weaknessess in NATO's conventional posture which have always plagued the alliance" - such as mobilizing quickly and in a coordinated fashion in an emergency.

NATO's central sector of the German front has "serious" but not "insurmountable" problems, while NATO's southern flank "can be regarded as little more than a shambles."

Despite the growing Soviet navy, NATO's Mediterranean members appear to be abdicating their responsibilities under the North Atlantic Treaty" as Italy cuts its army by one-third, Greece takes its troops out from under the NATO command, and Greece and Turkey display "no more enlightened objectives than preparation for war against one another."

Up north, where early reinforcement is more important than anywhere else along the NATO front, the repott said, the situation "is little better" because "neither Norway nor Denmark as yet appears predisposed to undertake the the initiatives necessary" to assure rapid reinforcement in the Warsaw Pact strikes there.

In contrast to Soviet forces, which could hit NATO troops from "a standing start," NATO nations would need mobilization time they are unlikely to get Warsaw Pact troops could well be into West Germany by the time NATO decided to use tactical nuclear weapons, meaning they would fall on friendly territory if fired, the report said.

Declaring that Soviet forces along the NATO front have grown in size and quality over the last eight years citing new generations of armor, artillery, aircraft and antitank weapons - the report said the threat to NATO "has fundamentally altered."

Warned the Senate report: "Soviet forces deployed in Eastern Europe now posses the ability to launch a potentially devastating conventional attack in Central Europe with little warning. Such an attack might be attended by the telitale priod callup" and redeployment of Communist troops to that sector.

Therefore, the report added, NATO "is confronted with the prospect of warning calculable in days, not weeks or months."

Asserting that "we are neither suggesting any intention on the part of the Soviets to invade Western Europe not predicting an invasion," the report said NATO nations must face the new facts of what the Soviets could do in a war and adjust their forces accordingly.

No longer is it an attractive option to use tactical nuclear weapons to stop an invader if conventional defenses collapse, the report said, because the Soviet Union's tactical nuclear weapons are now "more destructive and longer-ranged than NATOs."

Also, said the report, Warsaw Pact forces are better prepared than NATO's to fight on a battlefield where tactical nuclear weapons have been used.

The changed situation, the report said, dictates that NATO nations streamline mobilization procedures: strengthened NATO conventional forces along the NATO line especially the center, and move them closer to the front; improve their ability to communicate with one another; store ammunication, and more of it, closer to combat units, and change war planning to reckon with the liklihood of getting little warning of a Communist attack.