Unlikelihood Of Calibre Of Artillery Guns Going Beyond 155mm

Editor’s Note

The calibre of Artillery guns has stabilised at 155mm for quite a few decades, now. The US, NATO members and even the Indian army have all adopted the 155mm calibre. The standardisation of the calibre lends to better inter-operability and logistics among forces of allies. Further, optimisation of calibre being ruled out, it is improvement in ammunition, that remains the key to achieving greater ranges and effectiveness at the target end.

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‘Change is the only constant’ is the famous adage but change has been given the skip by the Artillery gun revolution. The Artillery guns have stagnated at 155mm calibre. It’s been a while since any new calibre gun has found its way into the Artillery arsenal. There have been improvements in the calibre lengths, like from 39 calibre to 45 to 52 calibre, but that too was some time back. Yet the calibre of the gun has remained 155mm. Will the Artillery guns see another standard calibre from the gun manufacturers or will the armies remain content with the current status quo. 

The evolution of Artillery guns has been a function of tactical needs, metallurgy and technology. The proliferation of Artillery calibres took off after WW II when the arms race gathered pace. The soviet bloc opted for the 122mm and 152mm calibre guns against the NATO block countries, which opted for 155mm calibres. Artillery guns have stagnated at the same calibre levels for over six decades. 

155mm The Standard Calibre

Why have the guns remained at 155mm calibre? The experience of WW II and the formation of two rival blocks necessitated a unity in thought among the members of the NATO alliance. The unity was in terms of fighting the perceived enemy together, in battlefields, which were mostly in Europe. Artillery was the first to lead this thought process. All guns were designed for a standard calibre with standard ammunition so they could be manufactured universally and used by all the NATO allies. Such a move ensured the rapid manufacture and movement of Artillery guns and ammunition to ward off an intended threat anywhere across Western Europe. 

Yet, this does not answer the fixation on 155mm calibre. Theoretically, calibre and calibre length (Length of the Barrel) determine the gun’s weight, size, shape and weight of the ammunition. The efforts for future development aimed to reduce the Artillery’s supply chain and increase its firepower. By the time the Gulf War II broke out, the operational doctrines had rapidly changed; degradation of depth targets and concurrent attacks on objectives in front and depth had become a norm. The new tactical doctrines required weapons which could provide intimate fire support and destruction at the same time. 

The new guns were able to deliver fire support up to 40 km. Where the ranges to targets were greater than 40 km, tubeless Artillery with ranges from 70 to 140 km was employed. Artillery support to infantry and armour formations could not be replaced as no other weapon could be as accurate, sustained, and varied as an Artillery gun. Thus, Artillery was the best instrument for last-mile fire support. By this time, the 155mm 39 calibre guns had given way to 45/52 calibre barrel lengths. A longer barrel added to the range achieved by the shell. By the time the 155mm 52 calibre guns came into service, the ranges achieved by Artillery guns was around 40 km.

Could Artillery guns be designed to provide close support and fire in-depth well beyond 40 km? That depended on whether there was a need for an Artillery gun to fire that deep. An Artillery gun/ howitzer is an expensive and complex weapon system, especially modern guns, compared to tube Artillery (Life Cycle Cost per unit). So, undertaking engagements of deep targets by tube Artillery was a more efficient option. Second, increasing the gun range would increase the weapon’s weight and ammunition system. The configuration of a 155mm gun had reached an optimum level, such that a slight increase in range would demand a considerable increase in size and weight, a parallel which could be best understood as improving the efficiency of a machine after it reaches its peak value. Every incremental change would require far greater effort and resources. The table below would explain the above.

Table 1- Showing Comparison of Guns

(Note: The data in the table is picked up from free sources on the internet)

The table above indicates that an increase in calibre does increase range. It could deliver larger explosive content to the target. Yet the rates of fire suggest that a 155mm can deliver a higher quantity of explosives on the target within one minute. It is the reason why the US Army discontinued the use of 203mm M 110 A2 in the US Army in the early 1990s. 

To understand the constraints of going in for higher calibre, a simple internal ballistic equation can prove the degree of difficulty involved. When moving from 105 mm to 155mm, as per simple calculations, the cross-sectional area of the shell will increase by 2.17 times approximately, indicating that, all else being equal, the larger shell could have more than twice the kinetic energy, giving it a potential for greater range. But when the gun’s weight is considered, from 105 mm to 155mm, the weight has increased by 4.38 times to 14 tons, while the range has increased by 3.5 times. It is possible to calculate hereafter for every increase in 10 mm calibre how the weight of the gun will increase. 

As per calculations, every 10 mm increase in calibre could lead to an increase of 2.16 tons in weight and 6 km in range of the gun on a linear basis. It will be substantially less due to the phenomenon of diminishing rate of returns. This mathematical explanation is an elementary representation of a complex interplay of factors such as the increasing rate of diminishing returns, the change in ammunition weight and propellant charge and the resultant recoil gasses and internal chamber and barrel pressure. All of these would increase the gun’s weight. Similar equations will play out for an increase in the weight of the ammunition. Thus, an increase in range and lethality would get into a steep upward-moving marginal utility curve along the Y axis, which represents weight, where any increase in the calibre of the gun more than the present 155mm would substantially lead to increase in weight and thus prove inefficient. 

Way Forward

Given the futility of increasing the calibre or calibre lengths, the way forward for artilleries worldwide, including India, is to concentrate on improving ammunition performance and types. Newer types of ammunition, like the rocket-assisted projectile, base bleed or the stabilised fins or nubs on the shell body, could improve the aerodynamic performance thus giving a better chance to achieve longer ranges. Improved metallurgy could lead to a higher threshold for sustaining pressure inside the barrel and could also pave the way for more optimum utilisation of propellant gasses. Apart from increasing range, developments in lethality and precision of Artillery rounds have seen great advances.

The R&D establishments are working overtime to improve ammunition performance more than increasing the calibre of the guns. 

Conclusion

Artillery guns have stabilised at an optimum calibre of 155mm and calibre lengths of 52 calibres and below, like 45 and 39 calibres. Developments in Artillery, if any, would be out of the gun/howitzer family in Hyper Velocity or laser guns, which would be a new stream. Both the demands of current tactical doctrines and the proliferation of long-range multi-barrel tube Artillery preclude any further design development in the gun-howitzer family. Indian Artillery has invested in the 155mm 52 calibre Dhanush Gun produced by the OFB (Ordinance Factory Board) as its gun for the future. The chances are that any further developments in Artillery would be driven by better ammunition rather than a new calibre gun. 

Maj Gen Nitin Prabhakar Gadkari (Retd)