A 155mm projectile fired from a howitzer reaches rotational speeds of several thousand revolutions per minute as it travels downrange. The rifling in the barrel imparts this spin to stabilize the shell in flight but that stabilization only works correctly if the projectile’s mass is symmetrically distributed around its geometric axis. Even minor imbalance, such as a deviation of the principal axis of inertia from the geometric axis, has a direct and measurable impact on flight trajectory and accuracy.
This is not an abstract engineering concern. The global 155mm ammunition market is projected to reach $6.2 billion by 2030, driven by expanding military stocks, demand for both traditional and precision-guided projectiles and the U.S. Army’s active development of next-generation rocket-assisted projectiles that are designed for ranges beyond 30 kilometers. As range and precision requirements increase, the manufacturing quality of the projectile body itself becomes a more significant factor in delivered accuracy — and balancing is the measurement and correction process that addresses it.
How Imbalance Affects Projectile Flight
When a spin-stabilized projectile has an asymmetric mass distribution, the result is a precession, which is a wobble of the projectile’s spin axis around the flight path. This precession:
- Increases aerodynamic drag — a precessing projectile presents a larger effective cross-section to the air, increasing drag and reducing range
- Introduces trajectory dispersion — the precession creates a lateral force component that deflects the projectile from its intended path, reducing accuracy
- Degrades precision guidance effectiveness — for guided projectiles like the Northrop Grumman Precision Guidance Kit (PGK), which uses GPS and canard control surfaces to steer the shell. An inherent body imbalance creates a persistent disturbance that the guidance system must continuously correct, thereby consuming control authority that would otherwise be available for course correction
The imbalance problem is particularly challenging because the projectile body is not a simple cylinder. The ogive (nose) shape, rotating band geometry, internal cavity for the explosive charge or payload and base configuration all create opportunities for asymmetric mass distribution — even in parts machined within dimensional tolerance.
The Measurement Challenge
Measuring the imbalance of a 155mm projectile requires a specialized approach. Unlike a conventional rotating component that operates continuously on bearings, a projectile spins only during flight because there is no shaft, no bearings and no opportunity to spin the assembled round at operational speed in a factory.
Cimat, an Ascential Technologies brand, has developed a dedicated measurement system tailored to the needs of 155mm ammunition manufacturers. The system addresses several unique requirements, including the following four items.
Initial and final unbalance measurement: The system measures the projectile shell’s unbalance before and after correction, providing both the magnitude and angular position of the imbalance vector. This enables precise, targeted correction with minimum material removal.
Classification of permissible final unbalance: The system applies a specialized classification of permissible residual unbalance expressed in multiple units. This includes angular deviation of the rotational axis, which is the most operationally meaningful metric for a spin-stabilized projectile. This classification directly relates the residual imbalance to its expected effect on flight trajectory.
Shell wall thickness asymmetry analysis: Through automatic data collection and statistical processing, the system analyzes the relationship between shell wall thickness asymmetry and measured imbalance across production lots. This enables the manufacturer to identify systematic machining trends and adjust upstream processes, which reduces the imbalance that reaches the balancing station in the first place.
Explosive charge influence compensation: The explosive charge or payload is placed inside the projectile after the balancing process is complete. The measurement system accounts for the influence of this charge on the final mass distribution ensuring that the balanced empty shell remains within tolerance after filling.
The Correction Process
Correction of 155mm projectile imbalance involves controlled material removal from the projectile body at calculated angular positions. The objective is to shift the principal axis of inertia to coincide with the geometric axis within the permissible tolerance band.
The correction must satisfy competing constraints:
- Structural integrity — material removal must not compromise the projectile body’s ability to withstand the extreme acceleration forces during firing — tens of thousands of times the force of gravity (g-forces) — and the gas pressure behind the rotating band
- Minimum material removal — each gram removed changes the projectile mass, which affects ballistic performance. The correction process must achieve the required balance with minimum mass change
- Production cycle time — ammunition production is high-volume; the balancing and correction cycle must be efficient enough to keep pace with the manufacturing line
Cimat’s measurement and correction systems are designed to optimize all three: achieving the required balance tolerance with minimum material removal and minimum correction time.
Production-Scale Balancing: Volume, Safety and Process Intelligence
High-volume 155mm production introduces additional requirements beyond measurement accuracy, including automation for cycle time, operator safety and recurring defect analysis.
Automation for cycle time: In a production environment manufacturing thousands of rounds, the balancing cycle (load, measure, correct, verify, unload) must be optimized for throughput. Cimat builds balancing machines designed for automated production lines with cycle times optimized for high-volume efficiency.
Operator safety: Ammunition production involves handling energetic materials and heavy projectile bodies. Automation of the balancing and projectile transfer stages — both within and outside the machining cell — is essential for operator safety, not just efficiency.
Recurring defect analysis: In high-volume balancing, Cimat integrates a system for analyzing recurring shell manufacturing defects. Based on analysis of unbalanced components across production lots, the system provides the manufacturer with guidance on machining parameters for subsequent units, which raises the quality of the entire production process, not just individual corrections.
This closed-loop feedback — from balancing measurement back to upstream machining — is where balancing transcends quality inspection and becomes a process improvement tool.
Why This Matters Now
The expansion of 155mm production capacity worldwide, including new producers entering the market, is creating demand for balancing and measurement infrastructure that did not exist at current scale five years ago. As production volumes increase and precision requirements tighten with next-generation guided projectiles, the balancing station becomes a critical quality gate that directly affects the combat effectiveness of the ammunition it produces.
Cimat brings decades of experience in defense-grade dynamic balancing to this application with measurement systems, correction optimization and production automation designed specifically for the unique requirements of ammunition manufacturing.
Sources
- Yahoo Finance — 155mm Ammunition Market Report 2026-2030: Global $6.2 Billion Industry
- The Defense Post — US Army Pushes Artillery Limits With Next-Gen Rocket-Assisted Rounds
- Northrop Grumman — Modernizing the Precision Guidance Kit
- Janes — WB Group to Start 155mm Shell Production in 2026
- U.S. Army — 155mm Round with Increased Range and Precision
