The big thing wrong with Pentagon contracting is the Pentagon, Deputy Defense Secretary Frank C. Carlucci said yesterday in releasing his most detailed battle plan yet for reducing cost overruns and other foulups in the buying and building of weaponry.

Carlucci, when Defense Secretary Caspar W. Weinberger has appointed field general for the promised war on military waste, complained that the Pentagon has not been able to make up its mind on what it wants; has pursued weapons that are out of reach rather than ones sure to work, and has force-fed itself and weapons makers so much paper that the procurement system is choking.

Acknowledging that reforms have been promised before by his predecessors at the Pentagon, only to evaporate, Carlucci said it takes "relentless action on the part of top management" to implement them. Weinberger and Carlucci are promising that.

His memo saying all this may have been released in hopes of reassuring the Pentagon's allies in Congress. They have been warning that the nation's current pro-defense constituency will break apart unless the Reagan administration demonstrates to taxpayers that it is eliminating waste as it spends record high amounts on defense.

In addition to that warning, Sen. Ernest F. Hollings (D-S.C.), ranking minority member of the Senate Budget Committee, is predicting that Reagan's $1.5 trillion program for rearming America will end in disaster unless the administration bases it on realistic inflation estimates. He contends that the fiscal 1982 through 1986 defense program is underestimated by $150 billion because of unrealistically low assumptions about the rate of inflation.

Carlucci made these points in elaborating on 31 decisions on procurement recently sent to Pentagon civilian executives and the military's Joint Chiefs of Staff:

Stop-and-go contracting. The biggest reason for cost overruns on 47 major programs was that the Pentagon kept changing its mind on how many weapons and how soon. Carlucci demanded that civilian executives set the pace and stick to it from now on. He complained that of the 47 programs that more than doubled over original cost estimates, orders for 19 went up, 20 went down and eight remained the same.

The better-mousetrap phenomenon.Ordering the the most promising looking weapon, rather than the most reliable, "frequently ends up with large cost increases and schedule slippages." Carlucci said weapons should be improved a step at a time and directed the Pentagon's designated under secretary of defense for research and engineering, Richard DeLauer, to come up with a plan for this within 30 days.

Low productivity. "Productivity in the defense sector of the U.S. economy has been lagging in large part because of the low levels of capital investment compared to U.S. manufacturing in general. . . . General counsel should support legislative initiatives to permit more rapid capital equipment depreciation. . . ."

Overcentralization. "During recent years there has been a growing tendency to centralize the decision-making process within the Department of Defense. This practice has multiplied throughout the numerous levels of authority in each of the services. . . . This practice has, in of itself, lengthened the acquisition cycle; created cost increases due to delays in decisions; confused the authority, responsibility and accountability of the designated services managers, and has stifled innovation which could produce program improvements leading to cost savings." Carlucci directed that authority be decentralized.

In the foreword to his memorandum on his 23 decisions on contracting reforms, Carlucci said "a primary objective in streamlining the Defense Department acquisition process is reducing costs. . . ."

Weinberger in a speech in San Francisco this week said "the old exhilaration I used to feel for budgetcutting is still there, and I hope to savor it often." He noted that he has appointed a deputy in his office to concentrate exclusively on ferreting out waste in the Pentagon.