The Interchangeability Crisis: Scale, Origin, and Operational Consequence

A NATO staff study (reported by Tasman (2026) and not independently corroborated in open sources at time of publication) examining five major artillery-producing member states has identified more than 60,000 theoretical weapon–ammunition combination permutations across Alliance land forces. Of these, the vast majority remain untested: not because they have been assessed and found non-viable, but because no systematic qualification framework exists to evaluate them. Many will physically chamber but are without validated firing tables, confirmed chamber pressure curves, or verified terminal effects data. The scale of this gap is not marginal. It is a structural deficiency with direct consequences for Allied operational readiness, logistical resilience, and force-multiplier effectiveness.

The origin of this deficit lies in three decades of post–Cold War budget contraction. Following the Warsaw Pact’s dissolution, NATO nations progressively reduced defence expenditure, force structures, and (critically) ammunition production capacity. Alliance membership doubled over the same period, yet ammunition interoperability received declining institutional priority. Standardisation activities that require sustained expert investment, testing infrastructure, and multinational consensus became casualties of the “peace dividend.” The result: a technically fragmented Alliance unable to reliably answer the field commander’s most fundamental logistical question: can I fire this projectile, with that charge and fuze, from this gun?

Russia’s full-scale invasion of Ukraine on 24 February 2022 forced an immediate operational reckoning. Ukrainian ground forces, equipped primarily with 152 mm Soviet-legacy artillery systems, rapidly received Western platforms, including U.S. M777 towed howitzers, British AS-90 self-propelled guns, French CAESAR wheeled systems, and German PzH 2000 self-propelled howitzers, each firing 155 mm NATO-standard ammunition. The immediate crisis was not calibre incompatibility between the Western systems, but the absence of validated firing tables for the specific projectile–propellant–fuze–primer combinations arriving from multiple contributing nations. Field commanders transmitted urgent requests to NATO headquarters, accompanied by photographs of unfamiliar components, seeking authoritative confirmation that the combinations available to them were safe and effective to fire. Those requests revealed, with operational urgency, how shallow the Alliance’s interchangeability architecture had become outside the small-calibre domain.

Technical Architecture: What NATO Standardisation Does, and Does Not, Cover

NATO’s ammunition standardisation framework is tiered by calibre and technical maturity, and the contrast between the small-calibre domain and indirect fire munitions is stark.

For small arms ammunition up to 40 mm, the Alliance has established a mature qualification architecture. The AC/225 NATO Army Armaments Group (NAAG) Land Capability Group Dismounted Soldier System (LCGDSS) Sub-Group 1 on Small Arms Ammunition Interchangeability (SG/1) governs standardisation, testing, and qualification of common infantry calibres: 5.56 mm, 7.62 mm, 9 mm, and 12.7 mm. Two NATO Regional Test Centres (RTCs), one in the United Kingdom and one in the United States, conduct standardised tests per AEP-97 (Multi-Calibre Manual of Proof and Inspection, M-C MOPI). Ammunition designs that satisfy all M-C MOPI requirements receive NATO Design Numbers and formal NAAG interchangeability declarations; qualified rounds are marked with the NATO interchangeability symbol on consignment packaging. The NATO Support and Procurement Agency (NSPA) Ammunition Support Partnership (ASP) limits standard small arms procurement to NATO-qualified designs exclusively: a procurement restriction that enforces compliance with the qualification architecture in a way that no STANAG alone can achieve.

The indirect fire domain presents a markedly less mature architecture. For 81 mm and 120 mm mortars and 105 mm and 155 mm artillery, centralised NATO qualification equivalent to the small-calibre model does not exist. Cost is the primary stated obstacle, the testing investment for large-calibre systems is orders of magnitude greater, but significant technical problems compound it: open-air test replication is difficult, and designating reference weapons for multi-national qualification is procedurally complex. Instead, STANAG 4425 (AOP-29) establishes a reporting framework whereby nations submit national test results and assessments regarding the safe firing of specific ammunition–weapon combinations. Critically, AOP-29 is not a NATO interchangeability certification document. It aggregates national data without providing mutual recognition or formal Alliance-level qualification. Member states routinely conduct their own confirmatory testing on combinations favourably reported by other nations, citing differing national regulatory requirements and absence of mutual recognition frameworks.

The data currency problem compounds this limitation severely. The current AOP-29 does not include systems developed since the mid-1990s, a gap spanning more than three decades of artillery and mortar development across nations that joined NATO after the Cold War. A revised digital “living” AOP-29, incorporating Ukraine operational data and new system combinations compiled through NAAG Integrated Capability Group Indirect Fire (ICGIF) Sub-Group 2 on Ballistics, Effectiveness and Fire Control Software (SG/2), was targeted for release before end-2025. Delivery status remains unconfirmed at time of ISC publication (March 2026).

Medium- and large-calibre direct fire ammunition, including 30 mm × 173 cannon and 40 mm case-telescoped (designated 40 mm × 255) rounds, sits in the weakest standardisation position. Existing NATO standards for 105 mm and 120 mm tank gun ammunition, which previously prescribed form, fit, and basic test parameters, were cancelled on grounds of obsolescence; the 120 mm smooth-bore standard, STANAG 4385, was cancelled in August 2016. NAAG Land Capability Group Land Engagement (LCGLE) studies are ongoing, but no replacement framework is yet in force.

The F3 and S3 Framework: What Interchangeability Actually Requires

NATO distinguishes two levels of interchangeability, each governed by a separate publication framework. The first is Form, Fit, and Function (F3): basic physical interchangeability, the item chambers, connects, and performs its intended mechanical role. The second is Safety and Suitability for Service (S3): full certification that the combination is safe to handle, store, transport, and employ operationally, with verified performance across the expected operating envelope. An item can satisfy F3 without achieving S3. Most of the 60,000+ untested combinations described in the previous section are precisely in this position: physically compatible, but without the testing and documentation that S3 certification demands.

Form, Fit, and Function (F3): Governed by AOP-6 Volume I (Catalogue of Ammunition Held by Nations That Satisfy Interchangeability Criteria of Form, Fit and Function Only), registered under STANAG 2928. The catalogue lists ammunition items from NATO nations meeting basic F3 criteria: type, mass, and geometric specifications for projectiles, propellant charges, fuzes, and primers. Physical dimensions and shape determine chamber compatibility, but registration in AOP-6 Vol I carries no performance or safety guarantee beyond that basic physical interface. A 155 mm projectile from one nation may satisfy F3 dimensional criteria while differing in rotating band geometry, boat-tail profile, or base cavity configuration in ways that affect obturation, barrel wear, and gas seal integrity. AOP-6 Volume II provides deeper National Interchangeability Documents (NIDs) where nations have conducted additional assessment beyond F3 basic criteria.

Safety and Suitability for Service (S3), Functioning and Firing Safety: Full S3 certification is governed by AOP-15 (Guidance on the Assessment of the Safety and Suitability for Service of Non-Nuclear Munitions for NATO Armed Forces), registered under STANAG 4297. For large-calibre indirect fire systems, S3 assessment covers: projectile structural integrity under gun-launch acceleration loads, fuze arm-safe performance (arming distance and setback threshold), correct propellant ignition pressure, and weapon system tolerance parameters including maximum allowable chamber pressure (P_max). Cross-combination variation in propellant type, charge mass, and primer sensitivity can generate chamber pressures significantly outside a weapon system’s design envelope, with consequent risks to barrel life and crew safety. National S3 assessments are conducted by authorised boards drawing on the Allied Ammunition Safety and Suitability for Service Assessment Testing Publications (AAS3P series), which provide standardised test procedures and templates by munition category. For small arms, AEP-97 (M-C MOPI) performs an equivalent qualifying role under AC/225 SG/1.

Safety and Suitability for Service (S3), Delivery, Accuracy, and Terminal Effects: The second S3 dimension addresses operational performance. Variations in projectile mass, centre of gravity, base drag coefficient, and muzzle velocity affect range, dispersion, and time of flight. For fire missions relying on computed firing data (particularly with guided and extended-range munitions) an uncalibrated propellant–projectile combination can produce miss distances that invalidate the tactical utility of the pairing. AOP-15 requires full qualification of terminal effects and ballistic data. The SG/2 Shareable Software Suite (S4) provides virtual testing capability to extrapolate from prior confirmed combination data, partially reducing the physical test burden for similar combinations.

Handling and Logistics: Ammunition and component compatibility with the full logistics chain from production through disposal: transportation and storage (including Hazard Division (HD) and Compatibility Group (CG) classification under STANAG 4439; Policy for Introduction, Assessment, and Testing for Insensitive Munitions (IM), and AOP-7 Edition 3: Manual of NATO Safety Principles for the Storage of Military Ammunition and Explosives), environmental sensitivity, fuze safety state during transit, packaging dimensions, and labelling. Both standards are managed by the NATO Ammunition Safety Group, CASG (AC/326), under the Conference of National Armaments Directors (CNAD). These standards generally apply Alliance-wide, but exceptions exist for munitions with non-standard handling requirements. Note that AOP-15 S3 life-cycle assessment also encompasses handling and storage safety criteria, meaning a complete S3 certification addresses all four dimensions.

Systemic Obstacles and the Procurement Dimension

The technical standardisation deficit sits alongside a set of structural and contractual obstacles that operate independently of the standards architecture itself. Defence industry contracts routinely restrict the use of alternative-manufacturer ammunition and components as a condition of platform guarantee and maintenance provision, a commercially rational position that directly conflicts with Alliance interchangeability objectives. Large weapon systems may require recalibration of fire control systems and full firing table validation before an alternative propellant or projectile can be authorised for use, creating a certification overhead that is manageable in peacetime but potentially prohibitive under wartime logistics pressure.

National legislation adds further fragmentation. Environmental regulations in some member states prohibit specific propellant compounds or heavy-metal projectile materials; alternatives may alter interior ballistic characteristics in ways that require re-qualification. Certification acceptance criteria vary by nation, and no mutual recognition framework currently exists that would allow one nation’s qualification data to substitute for another’s national approval process, except through the prospective NATO Ammunition Recognition Program (NARP), which addresses precisely this interoperability gap.

Batch qualification (verifying that each production batch conforms to the qualified design) is manageable in peacetime but can become a binding constraint on ammunition flow rates under wartime surge demand. The 32-member Alliance’s consensus-based decision-making structure imposes significant timelines on standardisation updates that require unanimous ratification, regardless of operational urgency.

In the UK context, cross-national ammunition use must additionally satisfy Defence Safety Authority (DSA) requirements under DSA 03.OME (Defence Ordnance, Munitions and Explosives Regulations) which places the national duty-holder obligation on the procuring authority regardless of Alliance-level standardisation status. UK procurement officers cannot rely on NATO qualification alone to discharge their domestic regulatory obligations.

The Remediation Pathway: Technical and Institutional Priorities

Tasman identifies eleven remediation vectors. ISC provisionally assesses these as falling into three priority tiers based on structural impact and near-term tractability, though the sequencing should be read as analytical judgment, not prescriptive programme guidance.

The highest-order structural fix is certification mutual recognition via NARP. Without it, testing investment by one nation produces no Alliance-level qualification benefit. Every downstream improvement (digital tools, multinational test facilities, forward pre-certified stockpiles) operates at reduced efficiency until a nation’s certification data can substitute for another’s national approval process. NARP is the unlock mechanism.

The most technically tractable near-term acceleration pathway is digital testing infrastructure. The SG/2 Shareable Software Suite (S4) already enables virtual testing and extrapolation from prior test data. Scaling S4 with machine-learning methods and expanding its validated dataset with Ukraine field data would reduce the 60,000+ untested combination backlog without proportional real-world test investment. Blockchain-based ammunition traceability would support batch qualification under surge conditions where current paper-based systems become rate-limiting.

The NATO Joint Fires Centre of Excellence in Slovakia, if resourced appropriately for large-calibre test work, addresses the shared physical infrastructure gap that constrains Priorities 1 and 2. Exercise-integrated ammunition swap programmes would generate actionable interchangeability data at Alliance scale, faster than any bilateral national agreement.

Accessing Restricted NATO Publications: The Role of MSIAC

The majority of publications central to this article (AOP-6 Vol I, AOP-15, AOP-29, AOP-7, the AAS3P series, and associated STANAGs) are restricted NATO documents not available through open channels. The primary institutional access route for member nations is the NATO Munitions Safety Information Analysis Center (MSIAC), a NATO project office funded and directed by its member nations and operating under CASG (AC/326).

MSIAC was established on 15 December 2004 following the merger of two former NATO bodies: AC/258, which had responsibility for the safety aspects of transporting and storing military ammunition and explosives, and AC/310, which oversaw the safety and suitability for service of munitions and explosives. That organisational history is directly reflected in MSIAC’s current scope: the centre sits at the intersection of the S3 (Safety and Suitability for Service) and storage/transport safety domains that are central to the interchangeability challenge described in this article.

MSIAC’s stated mission is to “eliminate safety risks from unintended reactions of munitions and energetic materials throughout their lifecycle.” It delivers this through technical consultancy, software tools (including the Munition Safety Standards database, MSaS), visits to member nations, workshops, and a publications repository (WebLink) that provides access to both open and limited-distribution MSIAC reports, NATO standards, and munitions-related conference proceedings. All services are provided free at point of delivery to government, industry, and academia of member nations.

Current MSIAC membership comprises 16 nations: Australia, Belgium, Canada, Denmark, Finland, France, Germany, Italy, Netherlands, Norway, Poland, Spain, Sweden, Switzerland, the United Kingdom, and the United States. Personnel from these nations can access restricted NATO publications (including AOP-6, AOP-15, AOP-7, and the AAS3P test series) via MSIAC WebLink, subject to eligibility confirmation through their nation’s National Focal Point Officer. Non-member nations and organisations seeking access to these publications should approach their national defence authorities or NATO Standardisation Office (NSO) through appropriate channels.