The Nunn-McCurdy Act: How Congress Polices Major Defense Acquisition Cost Overruns

The Nunn-McCurdy Act controls major defense acquisition cost overruns by forcing the Department of Defense to notify Congress when a program exceeds specific unit-cost thresholds, and presumptively terminating the program if the overrun becomes critical. Enacted in 1982, the statute operates as an automatic tripwire within the Pentagon's procurement system, shifting authority from program managers to lawmakers when a weapon system's budget deviates from its baseline.[1]

"For 30 years, the Nunn-McCurdy Act (10 U.S.C. §2433) has served as one of the principal mechanisms for notifying Congress of cost overruns in Major Defense Acquisition Programs," according to the Congressional Research Service. The mechanism relies on two specific metrics: the Program Acquisition Unit Cost, which divides the total cost of development, procurement, and construction by the number of units, and the Procurement Unit Cost, which isolates the procurement cost per unit.[1]

By measuring unit costs rather than total program costs, the statute prevents the military from hiding cost overruns by simply buying fewer aircraft or ships. If a program's overall budget remains flat but the quantity procured is cut in half, the unit cost doubles, triggering a breach. This mathematical reality means that a program can breach the statutory thresholds without any actual cost overrun in the engineering or manufacturing phases.[1][7]

The statute establishes two tiers of severity: significant and critical. A significant breach occurs when a program's unit cost increases by 15 percent over its current baseline estimate, or 30 percent over its original baseline estimate. At this stage, the Department of Defense must formally notify Congress of the overrun, triggering heightened oversight but allowing the program to proceed without immediate structural changes.[1][2]

A critical breach represents a more severe structural failure. This threshold is crossed when unit costs increase by 25 percent over the current baseline, or 50 percent over the original baseline. Under the Weapon Systems Acquisition Reform Act of 2009, a critical breach triggers a statutory presumption of termination. The program is automatically slated for cancellation unless the Secretary of Defense formally intervenes.[1][4]

To save a program that has suffered a critical breach, the Under Secretary of Defense for Acquisition and Sustainment must certify to Congress that the system is essential to national security and that no cheaper alternative exists. The certification process requires the Pentagon to revoke the program's prior milestone approval, restructure the management framework, and provide lawmakers with a written explanation of the root causes behind the cost growth.[1][2]

The certification process forces the military to justify its continued investment. In January 2024, the Air Force notified Congress that the Sentinel intercontinental ballistic missile program had exceeded its baseline cost projections, resulting in a critical breach. To continue the program, the Pentagon had to rescind Sentinel's Milestone B approval and direct the Air Force to restructure the program to control future costs.[5]

Congress has repeatedly amended the statute to close loopholes that allowed the Pentagon to evade reporting requirements. The most significant structural change occurred in the Fiscal Year 2006 National Defense Authorization Act, which added the original baseline estimate as a secondary threshold against which to measure cost growth. Prior to 2006, the military could avoid a breach by simply re-baselining a program's current estimate to match its inflated costs.[1][3]

Between 2007 and 2015, the Department of Defense recorded 37 Nunn-McCurdy breaches across its major acquisition portfolio. Of those, 13 were classified as significant breaches, while 24 reached the critical threshold. Despite the statutory presumption of termination for critical breaches, actual cancellations remain rare. Congress normally accepts the Secretary of Defense's certification that the capability is essential, though the process frequently results in reduced procurement quantities or management overhauls.[1][6]

The Space-Based Infrared System triggered breaches in 2002 and 2005, while the Navy's Zumwalt-class destroyer saw its production cost increase from a projected $3.15 billion to $5.82 billion per ship, triggering a breach and the cancellation of further production in 2016. In 2025, the B-52 Radar Modernization Program experienced a 17 percent unit cost increase, triggering a significant breach that required congressional notification but avoided the mandatory certification process.[6]

The statute's reliance on unit cost metrics creates a feedback loop in defense procurement. When a program experiences early development delays, the military often attempts to control total costs by reducing the number of units it plans to buy. This reduction drives the unit cost higher, which triggers a Nunn-McCurdy breach, which in turn forces a congressional review that can lead to further cuts.[1][7]

The effectiveness of the Nunn-McCurdy Act depends entirely on the accuracy of the original baseline estimates. If a program enters development with an artificially low cost projection, a breach is mathematically inevitable regardless of how efficiently the program is managed. The statute functions less as a tool for preventing cost growth, and more as a mandatory audit mechanism that forces the Pentagon to publicly defend its most troubled acquisitions.[1][4]