While the Reagan administration debates where to base the new MX missile, an editorial in a conservative military journal has sharply questioned the fundamental assumption upon which the $50 billion MX program is based.

That assumption is the official contention that the 1,000 existing U.S. Minuteman land-based missiles are vulnerable to being wiped out in an attack by increasingly accurate and numerous Soviet missiles.

The editorial by the military editor of "Strategic Review," a quarterly journal that has on its editorial board numerous retired admirals and generals and some civilian specialists associated with President Reagan, claims that "competent studies" show that neither U.S. nor Soviet missiles are accurate enough to carry out such a devastatingly accurate first strike.

The writer, military editor Arthur G.B. Metcalf, calls for either Congress or a special committee to look into this accuracy question, using "fresh technical expertise free of conflict of interest." It is, he says, "a subject which has too long been shrouded in secrecy."

"A number of issues having grave implications for our country hang upon this question of missile accuracy," Metcalf writes in the current summer edition, and "it is time we knew the full facts."

Metcalf makes it clear that he favors building the MX, but only as a replacement for the "aging" Minutemen, which were built mostly in the 1960s, and to match what he calls the Soviets' "capability for coercion." He favors putting the MX in the Minuteman silos, at a "saving of tens of billions of dollars," because he believes the idea of a vast land-based "shell-game" network of shelters to protect them from pinpoint Soviet attack is a huge waste of money that eventually might cripple the Air Force.

Top Pentagon officials in both the Carter and Reagan administrations have claimed that Soviet missile accuracy has progressed to the point where Moscow now has at least the technical and theoretical possibility to wipe out all but a tiny fraction of the Minuteman force. While agreeing this is an unlikely event, backers of the MX argue that it is only prudent to take that edge away from the Soviets.

But Metcalf argues that "this is not a substantiated fact, nor could it be. It is no more than agreed-upon position which has been handed to him the secretary of defense . Nothing has been put forward which technologically supports the belief that we (or the Soviets) could, with any degree of confidence, expect to hit one silo at ICBM range, let alone 1,000 of them distributed over an area equal to one-third of the United States."

Metcalf said he did not clear his article with the Reagan administration beforehand but that he consulted with many current and retired senior military officers and other specialists and that his views reflect a consensus of those he consulted.

Metcalf said that studies made available to the Townes committee, a panel appointed by Defense Secretary Caspar W. Weinberger to look into MX basing alternatives, should get wider examination. At least one of those studies, by a University of Minnesota professor, is known to have challenged the vulnerability argument.

"We do not know -- and according to inertial guidance authorities, we cannot ever know -- the extent to which a multiplicity of indeterminate fundamental physical forces render uncertain the point of impact of such missiles when fired for the first time in an untested trajectory," Metcalf writes. "This is not opinion; this is physics, for which analytical proof exists. No proof -- not even on a theoretical basis -- has been offered to support the 'accuracies' currently claimed. No refutation has been offered to the finding that Minuteman 'vulnerability' is without foundation," he says.

Metcalf's argument is that all of the "unpredictable" forces -- such as wind, weather, changes in the earth's magnetic field or variations in the pull of gravity at different points -- that can combine to throw a missile off-course can only be corrected by repeated tests with identical missiles over the same course and cannot be transferred to missiles that will be flying a different course to a real wartime target.